Comment by jroesner

Don’t know how it was done back in the days. But dumping a ROM should be straight forward. The resulting machine code can be massaged into assembly using a disassembler. 6502 assembly is kind of easy to read, since the instruction set is small compared to modern day CPUs. Once you identified a routine you want to change, you look up the instructions address in ROM, and hijack the corresponding addresses at runtime to put your own instructions and data on the bus. For sure everything but easy, but totally doable.