The cards in the main board are cards that I currently own/have in my deck on MTGA. Everything else are cards that could possibly be swapped in. That said I am having decent success with the mainboard cards already.

I have also had sucess dropping 2x/4x Mirror Image for 2x/4x Tattered Mummy. This just makes the deck a little faster and helps the curve a bit.

Cards you can sub in if you do not have all of the cards or, for whatever reason, just don't like some of them:
More Lord of the Accursed.
More Death Baron.
More Gravedigger.
Graveyard Marshal.
Opt.
Plague Belcher.  Generally swaped for Ammit Eternal but really swapping with anything works.
Doomed Dissenter.
Infection Horror.
Ravenous Chupacabra.
 

I haven't messed around with all of the cards listed here but I have messed around with most of them and I can say that they work pretty well for me personally.

If it isn't obvious this deck is meant to be an aggro deck. The most expensive cards in this deck being 4/5 cost cards (of which there are a total of 6) wich means you can reliably activate this deck with just 3 mana.

Due to how the 1st hand in MTGA is selected I chose to run only 21 lands even though various calculators (including the one on this site) will tell you to run more. If you get 2 lands in your starting hand you can be fairly certain that you will be able to play any card you pick up. If you get 3-4 lands in your starting hand you can absolutely play anything you start with. If you get more than 4 lands in your starting hand you might want to consider a mulligan.

Because of how Festering Mummy works you actually have a lot more board presence (early game) with these on the field. For example if you have a Festering Mummy and a Diregraf Ghoul on the field you can take on a 4 defense creature. You can also take on three seperate 1 defense creature.
If you have Plague Belcher you can also dump your -1/-1 counters onto Festering Mummy in order to deal 1 damage to the opponent and give one of their creatures a -1/-1 counter. Similarly if you use Plague Belchers -1/-1 counters on Tattered Mummy you can force your opponent to take 2 damage, though you will only ever want to do this after the combat phase. This also works with Fatal Push, Visious Offering and Never//Return for obvious reasons. I have won multiple games because I was able to kill my own Tattered Mummys.

Incase it isn't obvious the reason that there are so few Instant/Sorcery/Enchantment/Artifacts in this deck is because it makes proccing the damage portion off of the +1 on Liliana much more reliable.