will later gearbox fit

[patrickfromperth@yahoo.com.au]

I have a gearbox that wont shift through gears, its out of the bike. A good R100 RT 81 gearbox has come up for sale from a reliable source and I wondered if anyone knows if it would fit, and what mods might be needed.
I have R100 81 engine sitting in the shed so I could put these together later after my correct gearbox gets rebuilt, later as its at vast expense.
thanks
Patrick