> Do you figure out all the constraints of the target system, fork the original, adjust, and compile? Is it an involved process?
For most games, yes, but this particular game was written in assembler (as were virtually all games on that system), while the original game was written in C (probably with some x86 assembler as well, also incompatible with the console). So it had to be essentially a complete rewrite.
Writing DOOM in assembler sounds like a...gargantuan task. Is it a case where you can compare assembler output from the original or a start-from-scratch sort of thing?
Probably mostly a start from scratch. Even if you felt like disassembling and trying to grok the PC executable, it wouldn't be especially helpful - the SNES' 65816 CPU (and its SPC700 APU, and the Doom cart's Super FX coprocessor) are completely different than an x86; especially for a performance-intensive game like Doom, you'd likely need to rearchitecture the whole thing.
For most games, yes, but this particular game was written in assembler (as were virtually all games on that system), while the original game was written in C (probably with some x86 assembler as well, also incompatible with the console). So it had to be essentially a complete rewrite.