the 4.2 to 4.0 is the most direct swap the easiest way is to find a low mileage wreck in another jeep so you can get the harness and pcm depending on what year you get the motor from you can get dis (coil on plug) or distributor ignition you could rebuild your 4.2 , but by the time your done you have as much opr more in it than you do with a replacment or even a swap
256-4040 ask for derek or dave
Thanks