I have some old 120v tools that use that rubber coated wire including an industrial floor sander (the one used for hardwood floors) that I discovered had rotten wiring; wondered why it wouldn't work!
My theory is the way the motor was stored causes the disintegration, I have seen motors wrapped up with garbage bags or shrink wrapped trapping moisture and fumes.
If you examine a harness that still has the electrical tape on it the wiring is good - I use Mercury's Corrosion Gard on the wiring and metal parts.
This wiring diagram should help, the magneto wire grounds the magneto to stop - don't put 12v into it.