I do not have a schematic, but this reminds me of cars of yesteryear with a ballast resistor.
When starting, 12 volts went to coil, but when started, circuit went thought a resistor to reduce voltage to points and condenser.
If symptoms same as yours, ballast resistor failed.
What can you advise about 125 circuitry/coil?
Any mods to your stop/run switch circuit?