There's lots you can do to bump up power, and speed. First, and cheapest would be to purchase a larger pinion gear, and/or smaller spur gears. Each tooth BIGGER than your stock pinion will provide more top-end speed, but take a bit away from acceleration. Opposite rules apply with your spur. Smaller spur tooth count will increase top end.
If you want to spend a couple bucks, get a new motor. Not sure what to suggest, as I really don't know what's out there. But maybe put up a post, and ask a few guys what they'd recommend? Be sure to mention what you're using the truck for (racing/bashing/etc...) and what your desired outcome is. Do you want it fast? If so, how fast?