You'd be surprised.
The tracker is one of the most modified cars i've seen. People drop 350's into them with turbos and nos and blow away mustangs at the strip. Even the stock engine does well. Look on youtube for, El Megaton. Now that's tracker.
Anyway, there is a decent aftermarket base for these, but alot of it involves custom making everything for it, and they are usually expensive. Turbos, for example, are relatively easy to put on, granted you have to make all the stuff, but after it's on, you have to tune the engine to run on it, such as air/fuel ratio. Without it, it could drive worse than before, and if you don't know how to tune it you'll be paying quite a bit to have it done.
Here's a site that lists some of the simple ones:
Geo Tracker Modification Instructions
Hope i helped.
EDIT: Judging by the date of your post, i figure you proly sorted it all out. Maybe this will help another person reading it.