I've used similar flashlights in the past and got tired of changing and replacing batteries. Utilizing the engine to power headlights is the way to go, especially for night riders like myself.
You have to realize that there is not much coming out of the white wire. As people have said about 6 volts and it's also AC. Some have run a 6 volt 3 watt filiment bulb off of it. But since you need DC, you will only get half that power, about 1.5 watts.
That Cree is 5 VDC at 10 watts - 2 amps of current. You figure it out.
You could use a bridge recifier to make DC, but you will lose a volt and a half across the diodes.
I've made many offers on ebay, they usually go down a few bucks, sometime a few cents. I'm going to try a 5v led light and see how it goes. I'll report back.