Honestly, replacing the spring is a bit of a big deal, as it can't have too much tension (or too little), or else the head will spring back too quickly or too slowly, messing up the timing. The belt & gear positions inside also screw up the timing if you disassemble the whole thing. I hate the FDS.
Anyway, if you have a working FDS drive, yes, you can just replace the whole drive in the Twin Fami.