There's a couple of wires coming from a dedicated coil in the engine which seem to be part of making the genny actual generate. The coil in the engine would normally be a lighting or charging coil, but in this (archaic) install the wires go via a couple of diodes and over to the bridge rectifier in the genny control box. Without whipping the flywheels off my various spare engines I don't know (and doubt anyway) if any of them have lighting / charging coils fitted, and although I could source a coil to put in, it's far easier to make use of the chassis frame something else
BTW, there was a date stamp on one of the crapacitors of 1970.
Here's the wiring diagram from the genny. The wires in question are the two Y - yellow and one L? (light green) coming from 'Engine' in the bottom right hand corner.