I wouldn't bother. In theory it can be done from the engine, but only at 6 volts. I don't suppose that light would be all that bright.
When I first started I used an automotive headlight (55 watts) powered by a 14 amp-hr ATV battery. I'd charge the battery at home. The light was great, but I got tired of lugging that heavy battery around.
Nowadays, for motoring at night, I use four of those cheap little LED bicycle headlights.
I get them at Harbor Freight for 6.99 or something like that.
This is enough light at practically no cost or hassle.