do you want to upgrade the motor for more power or stay with the same size ? Jeeps are probly the vehicles with the most modified around, and you cna go all the way to a V8 ........many are going with diesels now.
I'm doing a stroker build with the factory inline 6. You can build one from 1000$ up to 5 or 6 grand. Make about 325 ft lbs for a basic stroker all the way up to competing with stock big blocks and desils. Check out your local you pull it type junk yard for Cherokees and wranglers. Usually find a stock running motor for 200$