need working headlights

Hi everyone, i baught a 1981 DT 175 Yamaha that was stripped down of all it's lighting it was used for just dirt racing. My problem is i want to get a headlight working on it so i can see at night if i have to. I tried an after market headlight it was more or less an offroad light for a truck, but it was for a 12 volt system, the bulb in it is a H3 55 watt 12.8 volt light, and they burn out within a minute of me turning on the light. Is there any way that i can reduce the voltage so that it doesn't burn out my bulbs? I'm running off of the yellow wire which i think is my magneto wire but i could be wrong, but i do know that the voltage rises as my RPM increases and i have no battery on this bike, so if anybody has any ideas how to rig up something that would work on the cheap end i would very much appreciate it.