Man, I had a Lunchbox like 15 years ago, what a blast they are to run! You're probably not going to find many Lunchbox specific hop-up parts, or even replacement parts anywhere other than e-Bay. The good thing is that a couple of the easiest and more effective upgrades still bolt and strap right in. Buy a set of bearings first of all. This'll get rid of the slop from the old bushings, and free up some horsepower. My personal opinion is that even a ROAR stock motor would provide more than enough speed and torque to make the Lunchbox a blast. If you absolutely HAVE to have more motor, look at a 19 turn spec motor. Keep in mind that you may have some tough to find parts on the vehicle, and more horsepower will increase the likelyhood that you'll break stuff. You may want to look at the parts for the newer variations of the Grasshopper - the transmission parts may drop right in.
In addition, pick up a few new battery packs. For sport use, 3000's are getting really cheap, and provide great power with tons of run time. Check out cheapbatterypacks.com. And if you don't already have one, an electronic speed control would vastly improve performance over the stock mechanical unit.