It is possible, anything mechanical/electric is possible if you spend enough time and money. But a TJ is a digital Jeep with numerous microprocessors in PCM, TCM, instrument cluster, etc. A CJ is an old analog Jeep. You will have to create an entire wiring harness and this will consitute a distinct downgrade in functionality, since you will no longer be monitoring all the sensors, controlling emissions, etc.
You will also need an old analog engine with no smog stuff, no electronic ignition, no electronic fuel injection (carburetor only), plus either a manual transmission or an old dumb auto with no TCM microprocessor. After those changes, you won't legally be able to register the Jeep to run on the road, it will be offroad use only.
I would estimate a $5000 budget for this. But it can be done.
