You can put a header on, jet chip, high flow cat/exhaust, "intake"... ETC. All the "pop" mods that people are into these days, but they don't do much to a stock motor. The main limiting factor is the head (valve size and porting), compression ratio and cam selection. You can spend hundreds of dollars and get maybe 5-10 HP (big maybe). If you have access to a shop you could build a stoker or just build up the motor a bit.

When I rebuilt my motor I bored it over cut the head and deck height and ported the head a little bit. I can beat just about any "pop" mod wrangler of the same vintage in a street race. I'm still running stock valve size and cam and intake and exhaust plumbing. I still pass smog with flying colors and don't have to worry too much about running to rich and scoring the cylinder walls or an inferior filter passing too much dirt.

If you do want more power take it to an engine shop. In the end I think the "pop" mods just ware the engine out sooner by letting dirt by and too rich a mixture. In the end though, itÆs a heavy 4x4 that isnÆt shaped too well for speed.