I'm just finishing my street-legal conversion for my 1986 KDX200. Back then Kawasaki didn't use either a regulator OR a rectifier! What they did was to use a very puny 6 volt AC lighting coil mounted inside the flywheel. Such a system allows the AC voltage to vary all over the place depending on RPM and electrical load. Lights off, it will make some fairly high voltage. But with a headlight bulb load, the voltage drops rapidly.
I bought a "full-wave" bridge rectifier from Radio Shack for ~$3 and a small 6 volt sealed lead/acid battery. The coil will now charge the battery if the revs are up a little, while the battery helps prevent high voltages from burning out the bulbs. I am just now testing this setup, and I may find I still need a voltage regulator. If so, Radio Shack has them pretty cheap.