Guys

Thesee bikes are all very old but the main components don't usually fail. Older electronics were quite robust, however the push connectors and insulation on individual wires can corrode or perish quite badly in 20 years. I would make the assumption that the components are ok and the problem is somewhere in the wiring loom itself. I had the same problem and it was caused by a corroded block connector causing the loom to earth out.

Hope this helps