1) Very likely a 1984 gearbox does not fit without changing the clutch assembly as well (to the later lighter flywheel design), you have to customize the gear shift lever as well as the shift linkages are different
2) Doable, but needs modifications to the frame for the upper shock mount, there might be other limitations I am not aware of
3) If you want mag wheels go with 1985 rear end from a 1985 Monolever R80RT or something, that is plug and play except for issues mentioned under 2). If the bolt pattern matches to a 3-lug monolver drive, it might just work
4) Be aware of rake angle and height: again, a 1985 monolever front end can be used, dual discs and brakes are half decent with options to modify/upgrade
You can go much wilder and install a 1988-on paralever drive with a K1/K1100 disc brake rear end and front K100 forks. I have not seen the K rear end mod in Northamerica (it is quite common in Europe), Lornce did the K100 front forks mod on his R100.
I personally like this BMW-Ducati hybrid with K1 front forks, a K100 rear end and custom made swingarm (based on a monolever) with two exhaust cans under the seat:
Almost everything can be done.