The troubleshooting steps in the manual hits on most of the issues. Most often it seems to be either a bad regulator or broken wiring to the coils. Grab a multimeter and start there. Working backward: the AC voltage between the white wires and ground should be around 15 VAC while running, if that is good check the rectifier. Resistance between white wires should be about an ohm, and resistance through the field coil should be about 4 ohms; if not check for bad wires under the sprocket cover, otherwise a wire is broken inside the engine. Bypass the regulator by giving the field coil unregulated power: connect the field wire directly to power or ground, depending on how the regulator is set up (check the wiring diagram), if the voltage doesn't change (while running) go back and double-check the components and the wires between them. It's not difficult, but can be tedious.
