If the crankshaft keyway is a bit worn the key has a tendancy to slide out when you shove the flywheel on but if you set it so the key is at a slightly bigger angle than the crank taper, the flywheel should slide over then shove the back end of the key down as it goes. As far as I know, the shaft drive Big Red three wheeler was 84 to 86 and the four wheeler 86 only.

 

 

Doubt if you will get one. Is it the exciter or charging coils that have failed? Either way, you can get the coils that have failed re-wound, if new ones are not available.