How Much is it to replace the entire engine?

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.