You'll need precise and complete documentation of the CPU and other hardware, which is not too hard to find nowdays (17 years ago it was mostly books and they were incomplete/wrong).
You can probably go a long way just by programming by the specifications with some literature on designing interpreters, but good assembly language skills on the emulated system will be extremely useful, because that's what you'll be looking at all the time while debugging.
You can probably go a long way just by programming by the specifications with some literature on designing interpreters, but good assembly language skills on the emulated system will be extremely useful, because that's what you'll be looking at all the time while debugging.