How Much is it to replace the entire engine?

Autozone has engines available here http://bit.ly/16BfBi and the labor to remove and install the long block is 12.8 hrs and the radiator estimate is available on the repairpal estimator

Probably about $3,000 - $4,000 if you pay a shop to do it. If the engine overheated, it's probably just a warped head. Those heads will swap compression between cylinders 2 through 5 and run horribly. I've never seen a Jeep inline six warp the block from overheating. The only way to tell is to pull all six sparkplugs and run a compression test.