Actually, it's not that simple! Better to find an actual 999 but unfortunately they are as rare as hen's teeth. I looked for two years to find one.
You can put a low gear set into an earlier 904 but that's about it for later parts going into an earlier trans. The real beauty of the 999 is the 5 clutch drum but to install one in a regular 904 requires a lot of careful and hard-to-find parts matching.
Read
here.
