!!!MOM!!! Craigslist is still being stupid!!!
Some engines had "starter generators." These are typically on small engines, like some older garden tractors. I had an IHC "Cub Cadet" garden tractor for a little while, and it had a starter generator. In reality they simply have a starter solenoid which applies battery power to the armature, IE bypassing the cutout.
When I got it, the cutout relay in the regulator was bad, so I merely replaced it with a diode
The cub:
Tearing the bad cutout relay apart
I actually used a bridge rectifier because RadShack didn't have a diode I figgerrrrrd was large enough. The way this bridge is used here, two of the 4 diodes in the package are in parallel for more current
Wiring for a similar rig. No. 43 is the starter switch, and as you can see, it simply applies battery power to the armature of the generator, turning it into a motor