I'm not a skilled hacker (actually I'm not even a hacker), but it seems like a task I would be up to.This one might work.
Can you show me a tool I can use to disassemble the UMK3 ROM?
Do note that it is not specifically tailored to UMK3 (or any other game, for that matter), so you're going to have to manually identify all the code and data blocks, e.g. which routines and data actually do what. There is no "magic bullet" when it comes to disassembly, unfortunately.
Honestly, though...if you don't even consider yourself a hacker, I'm just gonna be blunt and say that this is not
an ideal first project to tackle (or even an ideal first console). A project of this scope would likely be daunting to even the most experienced hackers, given the sheer amount of work involved. You'd be much better off starting simpler (e.g. the NES) and working your way up from there. Read tutorials, learn 6502 ASM, talk to other hackers, and most of all: experiment! FCEUX 2.2.2 has an amazing set of debugging and hacking tools built in; I would suggest you jump in and play around with those to really get a feel for what this ROM hacking business is all about.