Well i can tell you what I did.
250 watt, 24 volt scooter motor from Allelectronics. run via #25 chain from the engine on one bike, and using the same set-up on the other bike only with a belt.
You have to adapt the metric shaft of the motor to whatever drive you use. This set-up is cheap and exceedingly reliable.
Actually, any DC motor will work, but the scooters have ball bearings and will last a long time.
Driving them at 1/2 speed, this is with 4 cycle engines that do not rev like a 2 stroke, the lights are dim at idle and bright at full rpm.
We can get away with not blowing lights by limiting the voltage to about 17 volts at full rpm. No regulator is needed. I went this way because in this state, lights are mandatory.
Both of these deals give off good light if the engine is revved some- like at cruising speed. Am using regular bulbs for like stop or turn lights, but these scooter motors would power up bigger lights easily.