Panelit,
I'm not sure there are fancy lights available for a 6 volt system, but using a headlight relay that was fed by a nice large gauge wire (fused) straight from the battery might perk them up somewhat. Mount the relay close to the lights and switch it with the current feed from switch to lights.
I think there is a thread in this forum running now about higher candle power taillight bulbs...dig around a bit and see what pops up.
Stuart
