What about a magneto? Then you can ditch the battery.
Pete2, a magneto would probably weigh as much as a small battery and then it has to be driven, consuming power.
Turbo, I'm not that familier with the 500s but with the 750 you can remove all the starter gear, the generator and the end of the crank were it was all located.
Another quick tip if you hadn't allready thought of it, run as thin a chain as posible along with thiner sprockets. Make sure it is a good quality race chain with no O or X rings. (the size modern day 250s use).
Sam.