It would help to know what engine you have but if it's a Chinese 2 stroke, you don't have many options.
As long as the lamp doesn't draw more than about 2.5 watts. That's all you'll get from the white wire/magneto, and it won't be much light. If the lamp draws more it will kill the ignition and your engine won't run.
I'd suggest mounting a SLA (sealed lead acid) battery, something around 5 to 7 AH (amp hour) and 12 volt to run your light. Recharge the battery after an evening ride.
I carry a 7.5 AH battery and I have a 55 watt sealed beam headlight. I can get a full hour of good illumination. I also have two bikes with 35 watt headlights. I can ride for close to two hours before the light starts to dim. Two hours is a lot of riding, especially at night.
Tom