For your 1.3 L engine, you can put a Thorley Header, MSD Ignition, Isky Cam, K&N Filter, 2'' free flowing exhaust and a carb that works better than your stock one. And if you are running bigger tires than the stock ones, change your gears if you did not (see Petroworks for reference). You also can put a 1.6 L engine in from a Sidekick/Tracker and they sell adaptors to make that fit for the Samuarai. And put all the performance parts in what I said before in a 1.6 L engine, and I think you might see some performance gains =). Calmini sells a lot of engine peformance parts for the Sidekick/Tracker.