What is wrong with the motor? If your replace everything on the motor, like harnesses and everything, make sure it's the HO 4.0 instead of the renix. Also stay away from the 00-01 for engines. Head cracking issues is to likely and they have coil packs, distributor is better. If is just the block any will work. Also don't try and swap a WJ or TJ motor in. It's a completely different block that requires some major mods to get it to all work.
For tires at stock height I would go with 235/75/15. Its the biggest tire size that you can fit at stock with no major issues. They are a tad bit bigger than the stock tire from the factory but work great on the XJ.
1997 XJ Sport
6.5" with 31's
Built to CRAWL