If you have a few thousand dollars you could rebuild, spercharger or turbo. Gears will help the power get to the wheels, but that has nothing to do with getting more power out of the engine.
Most of these can be done rather inexpensively with parts sourced from the junkyard.
After some research, this is what I came up with;
- Electric fan (Taurus)
- Cold air intake (XJ/TJ 4.0 tube and a cone filter)
- 4.0 Throttle body (XJ/TJ 4.0)
- 1 inch TB spacer (can be made from aluminum and a hole saw)
- headers
- 2.25 exhaust tube and high flow muffler/cat
- inertia ring