I imagine that you could use almost any kind of bulb with a little enginuity. If you use a Halogen bulb, make sure your power supply gives the bulb the required voltage.

I also get the impression that Xenon bulbs (I think that's what they are) are pretty bright. I have a lantern that uses a Xenon bulb, and it gives long battery life and a fair ammount of light. Although, I don't know how well it would perform on the road in real life conditions. It's something I've been considering experimenting with.

But Halogen bulbs are a pretty good bet if you want to upgrade. Those in conjunction with Nimh batteries should do nicely. Also, make sure that the light unit can withstand the heat.