We were lucky enough to find a debug version of Fatal Racing. Having function names helped a lot. That made the process much easier.

I have been less involved with the actual decomp in ROLLER specifically. Though I was part of one of the key decomp pieces which was demystifying the games binary storage format. That part alone took me a year of on and off work.

Once that was done, the rest of the process was much easier.

I suggest if you want to have a go yourself, you just need to start brute forcing it with IDA or Ghidra.

Additionally find out what compiler was used. If it was watcom, there is a great tool called wcdatool that may be of use to you.