Adding AI to a game without any is tricky; making AI is hard*, ROM hacking as far as doing extra code is hard, the meeting of the two does not make things any easier, especially if the AI is supposed to be good and done in real time by the console itself on top of everything else.
*various things to look at here. https://www.youtube.com/watch?v=DpXy041BIlA
is a good start even if it is for chess, all the things fighting game makers do by inputting events that happen in tournaments as training data, and stuff like the smash brothers neural network AI from a couple of years back ( https://www.engadget.com/2017/02/26/super-smash-bros-ai-competes-with-top-players/
) being interesting to look at.
Said neural networks might also be a choice option here, however that is possibly a bit beyond the scope of what we do around here; we could help with finding scoring items and some general baseline rules if you wanted (more rules is harder to train AI for but we can skip that one), but how many are familiar** with neural networks is a different matter, not to mention the general focus on having things run on hardware. It is not my thing though so there might already be such a project out there for some things that you could look at and twist to this.
**edit. I quite like https://www.youtube.com/user/carykh
as a blend of slightly lighter technical and demonstrative AI/neural network generation.
However if the game already has AI then in most cases it is reasonable to force a given character, stage or slightly altered rules (classic two rounds to win being nothing too major). For some games then the combo of levels, huge boss monster and such might see it. Difficulty changing can be harder (there is a reason many times bosses are given more health or cheap moves) and for the most part will come down to altering timings and occasionally giving the player a chance to recover or a making the AI occasionally flub things.