4.2L in a 92 XJ

How hard would it be to swap out a 4.0L I6 in a 92 XJ Jeep Cherokee for an older 4.2L I6?

Having problems with my current engine and would like to do away with the computer system and go back to a naturally aspirated engine.

Has anyone done this and what would be the major problems I would run into.

Thanks

It would no longer be street legal anywhere in the US, would have less power, less reliability, and less fuel economy.

Why would you do that to yourself? Do you like to walk?