You can use a 65C816 Disassembler to translate the binary ROM file back into ASM code.
Be warned however that a Disassembler generally cannot tell code and data apart and will try to translate everything into ASM code, regardless of whether it actually is code or not. As far as I know, there is no easy step for fixing this, and trying to sort out which parts of the ROM are code and which ones are data is a lot of tedious, manual work.
After that, you still need to understand how exactly the game is actually programmed and how it works internally, which is probably even harder.
Not saying it can't be done, but it will probably be really difficult and really time-consuming. Of course, knowing 65C816 ASM is vital too.
Best of luck to you.