It depends on what you're trying to light.
Most HID's require more current starting up than what the stator can handle, even if it's only a 50W HID.
On top of that, the stator doesn't produce a full charge at low RPM, so as soon as you drop down below a certain RPM needed to maintain 50W, the ballast in the HID might be damaged but at the very least it will likely stop working.
For example...you're ripping through the trails and come across a technical section (a washout or tight spot) that you stop for a second to find the best way through...at that point your HID stops working.
However, an LED light doesn't have those problems.
If they draw 50W, that's all they're going to draw regardless.
If they have reduced power (idling) then they'll simply dim.

Either way, a battery is a good idea, even if it's just a small one.
You can find some really small lifepo4 packs that you can literally clamp to the frame and they'll have more capacity than a motorcycle battery.