What exact change are you thinking?
'Scott', who hangs out on RGP sometimes, has done extensive modding of firepower specifically, even going so far as to rewrite the entire game as a system 7 game to take advantage of the expanded hardware capabilities, he's definitely the way to go regarding your change, if it happens that he hasn't already made it
I'm not aware of anyone else really doing system 6 modding these days. I've been working on a custom rom for sys 3-6 (https://pinside.com/pinball/forum/topic/re-rule-hot-tip-w-me) but it's also a total rewrite, not just a tweak like you're looking for
Seems like that forum is pretty much dead
It's too bad as rom hacking seems like an under used part of pinball. So few people do it much and there's no real place to put info though that it sorta stagnates. Would be cool to have a pinside subforum for it