Muktol

[[Marauding Blight-Priest]] can turn you life gain into life drain. [[Vault Skirge]] is a good early beater with evasion and lifelink.
If you don't need the lesson-cards that you're able to get using [[Cram Session]] then [[Pristine Talisman]] or [Trespasser's Curse]] could be a replacement.
[[Witherbloom Pledgemage]] could be replaced with [[Ill-Gotten Inheritance]] tough this would loose you another creature

Metalcraft cards could work fine here: [[Carapace Forger]], [[Galvanic Blast]], [[Mirran Mettle]]

Congratulations, and may all of you have a long, healthy, life!

Thank you hipponox for the information on [[Flamewave Invoker]], I did remove [[Kaervek's Torch]] to add the Invoker, and uped the number of [[Goblin Matron]] by 1. I'm not so sure if you really need 4 [[Skirk Prospector]] and 4 [[Flamewave Invoker]], especially when using [[Goblin Matron]] to search for the and [[Unearth]] tu bring them back from the gaveyard.
No, I did not overlook [[Flamewave Invoker]], I was searching for a different wording (deal damage all creatures)

The [[Shred Memory]] was/ is in the maybeboard due to it's transmute ability. It would be able to search for [[Putrid Goblin]] and especially [[Rolling Thunder]].

[[Mechanized Production]] or things like [[Broodstar]] (affinity, power/ toughness equal to the number artifacts you control) as finisher?

[[Helm of the Gods]], [[Nettlecyst]]

The title you are using suggests that this deck is intended for MTG Arena. (https://magic.wizards.com/de/mtgarena)
Arena allows the use of cards that date back to Ixalan (https://mtg.fandom.com/wiki/Historic_(format)).
The cards I mentioned ([[Soul Warden]], [[Viridian Longbow]], [[Lifelink]], [[Presence of Gond]]) all date back way before Ixalan.

*edit*
I just looked up the cards on Scryfall, and if you look for the historic legality, then [[Soul Warden]] and [[Presence of Gond]] seem to be legal and usable in arena, while [[Midnight Guard]], [[Lifelink]] and [[Viridian Longbow]] aren't.

With which version was [[Soul Warden]], [[Viridian Longbow]], [[Lifelink]] and [[Presence of Gond]] released for MTG Arena?

So I'm a fillet [[Nyx-Fleece Ram]] now...great. Muktol is a Tauren warrior! :)

I have tested a vehicle deck myself two weeks ago (https://www.mtgvault.com/muktol/decks/wu-pauper-vehicles/), but the main issue was that if I wasn't able to crew my vehicles, I wasn't able to do anything. Pretty similar to an equipment deck where you removed the bearer of all those weapons. I only used 16 creature spells, and perhaps to many vehicles, so that might have been the issue there. The problem with Pestilence is that it can exactly do this: Take out all those pilots you have, it doesn't need to get rid of the strong vehicles (And hell, if MBC wishes to do so, it still has a ton of "normal" removal).

I'm using Pestilence in 2 decks: MBC and dimir mill. Yes it's uncreative, unexciting, and you have to like the play-style, but if you look at how long MBC has been a strong deck, it is a deck that has to be taken into consideration in my opinion. That's why I did choose it a benchmark decks to test against.

99% of the decks I have built are kitchen-table/ fun brews. But for me it's more interesting to play a game that come out close, where each player stands a fighting chance, rather than having a deck crush another one, because it's unbalanced and has huge weak points. This is why I did choose to test all of my new pauper decks (and most of the old ones) against those benchmark decks. Not to make them more competitive, but to hone out weak points and in the end make each game more fun.

Boros monarch and mono black control. In my opinion monarch simply takes to long for aggressive decks to generate real value, they're better of with some simple card-draw. MBC keeps the opponent under control with a ton of removal, and when [[Thorn of the Black Rose]] it can even get more answers to problems in their hand. Boros monarch is a deck where all is about card-advantage, so [[Palace Sentinels]] fits in very well, and from what I've tested it, it's not really a fast deck.

The only thing that will safe you from a [[Pestilence]] is [[Prismatic Strands]]. [[Pestilence]] is always a problem when your opponent runs some high-toughness creatures, that allow his [[Pestilence]] to survive and be re-used. Boardswipe each turn is no fun at all. And if you loose your pilots, your deck is a sitting duck.

If you're interested, I can run some test against the benchmark-decks I used to test all of my own pauper decks (Boros Land Control, Dimir Defensive Mill, monoB control, monoG stompy, Orzhov lifegain/drain, Selesnya Tokens)

If you want to play singleton, you will have to find replacement for [[Sage's Row Savant]] and [[Wall of Lost Thoughts]], as you're using 2 copies of this card.

Problem with [[Fate Transfer]] is that it only works on creatures, and there is no creature in this deck, that uses any counters.

With 90+ decks on your account I wouldn't call you a starting player anymore ;)

[[Golem Foundry]], [[Everflowing Chalice]], [[Wing Splicer]], [[Karn's Bastion]]
Maybe [[Guildpact Informant]] or [[Flux Channeler]] instead of [[Skyship Plunderer]]

If you build new decks a deck-description would be a nice addition. This would help to make suggestions that will fit to your deck. You can also set the decks legality with the pull-down menu on the top right (Deck settings), or add some additional deck-tags.

[[Jor Kadeen, the Prevailer]] is on the high side mana-whise, but provides your vehicles (and [[Hotshot Mechanic]]) with a +3/+0 bonus.

+1
[[Bloodthirsty Aerialist]] instead of [[Wall of Limbs]]? Costs the same mana but can be used more aggressive than the wall.

Some other suggestions:
[[Curse of Death's Hold]] (Costs 1 mana more to play than [[Night of Souls' Betrayal]] but only affects your opponents creatures), [[Torment of Scarabs]], [[Accursed Witch]], [[Lampad of Death's Vigil]] (In combination with [[Sanitarium Skeleton]])

Deck and how-to-play description is missing.
I guess you know it, but I just wanted to point out that [[Wild Defiance]] does not trigger heroic on your creatures.
Are the decks you face that artifact heavy that you need to play [[Natural State]] mainboard? Also [[Silence]] seems a little bit off for an aggressive deck-archetype like heroic.

Suggestions:
[[Defiant Strike]], [[Emerge Unscathed]], [[Ethereal Armor]], [[Hyena Umbra]]

+1
[[Ill-Tempered Loner]], [[Brash Taunter]] or [[Pestilence]]?

+1/+1 counters always lead to the proliferate ability for me, so here are some suggestions:
[[Contagion Clasp]], [[Courage in Crisis]], [[Evolution Sage]], [[Planewide Celebration]], [[Urban Daggertooth]], [[Ring of Kalonia]]

Copying a spell doesn't count as casting a spell tough. What about phyrexian mana spells like [[Gut Shot]] or something with flashback like [[Lava Dart]]?

+1
First thing that came to my mind was "But I did use [[Lightning Phoenix]] in a deck that I stated as modern legal". So I raced of to check the deck....and had to replace [[Lightning Phoenix]] with something else, because I was to tired/ dumb/ whatever to read when building the deck. I did replace it with [[Chandra's Phoenix]] in my deck.
But as this wouldn't work out too good here I did a quick search and found [[Arclight Phoenix]]. Casting 3 spells should be no problem, especially when you don't have to pay for [[Lightning Phoenix]] any more.
Personally I would reduce [[Volt Charge]] to 2 cards, because you will only need them when you start "combo" with [[First Day of Class]] and bringing back your phoenix. With all the learn spells you can discard any unwanted cards and draw another card for it, so changes of finding [[Volt Charge]] even with a lower number in the deck are .