Sometimes the wiring through the frame is frayed going to the taillight it's usually best to bypass the neg ground return. It could be a ceramic resistor in the tail cracked. Its hidden behind the reflector block.
Yours should have the newer casehalve that will take a cdi upgrade. Recomended. You need the 4 mounting pylons some have just three.
The regulator ground is isolated from the rest of the grounds? If it isn't it will blow out bulbs and such if it's all on the same ground it would short it so it wouldn't start bad killswitch would do the same recheck the wiring try to simplify the problem check all the simple stuff spark fuel air ...