Usually the radio shack stuff is 9.6volt junk. If thats the case, the easiest way to "supercharge" it is add more batteries.
I had great luck with adding 3 nicad AA batteries to a radioshack car. if you can find a place to mount a 2 or 3 AA battery holder on the truck and do a tiny bit of soldering you'll notice a sizable difference in speed and acceleration.
Be aware that this truck likely has no bearings. The increase in power will greatly reduce the life of the cheap plastic bushings. when they get worn out enough the gear mesh will change and the gears will strip. You'll have a bit more fun with it before it before it dies though.