Using the d20 system ability modifiers instead of the B/X or OD&D modifiers in Shadowdark is certainly a choice.
One that I think is really quite excessive. It's making rolling 3d6 in order much harsher, and as wizard or priest, you need to make an Intelligence or Wisdom check every time you cast a spell. It seems much more grueling than oldschool D&D ever was.
@Yora Yeah, and failing those checks as a spellcaster is extremely harsh. The first spell a wizard at my table tried critically failed, causing a roll on the table.
He ended up getting spell drain/burn/whatever they call it and used up all his spell resources because he couldn't save to stop the effect.
Like yeah bad rolls suck. But the fact it was even a thing that could happen :/