ECU questions

Hi question is mainly for a cousin of mine, he has a 94 Gsx with too many mods to list, but the car could never get tuned correctly as well as run good, now his car came from california, and I was thinking maybe we need to change the ecu because of the smog and emissions restrictions, but from which years will an ecu work on his car, not looking for en eprom, just a normal 4g63 turbo ecu. What would work? Thanks