Offhand, I'd say that your problem is the votage regulator, but rather than start replacing parts to try to solve the problem, do this: Take a volt meter and test your lighting coil under the flywheel. You can find the procedure and resistance specs in any service manual (perhaps somebody else can rememember the spec).
If the lighting coil passes a resistance test, check all your wires, connections, and the handlebar switch for rust/corrosion. Be sure the ground wire at the voltage regluator is tight. If the light doesn't burn any better after all this (still not getting 12V), time for a new regulator. Hope that helps! Remember that you won't get 12V to the light (or tester) with the engine idling, even if everything is working properly. You'll see 7 or 8 volts until you rev it up and the coil can put out more juice.