You must have a HUGE motorcycle to have a mains generator on board!
I think if you measured 200V it would have been without any load, with the light switched on it would be much lower. In any case, what you are asking for depends on other conditions, in particular the current rating of the alternator and whether you have a battery in the circuit. What are you trying to achieve? If you want constant brightness, even when the engine is at low speed, it will not work. If the generator cannot provide enough power at low revs, no regulator will improve matters.
Brian.