If I were going to attempt something like this, I'd try to get the IP from a turbo 603, disable the 606 flaps and get rid of the engine computer. I'd also not attempt to put a 722.6 transmission into the car.

Transplanting the engine computer is asking for trouble. For one, as I already said, other modules communicate with it and some of them know the VIN. One of these modules receives the IR input from your key, so that will no longer work. You may be able to get the RCL to work the locks without the engine computer, and hack up some way to shut down the engine.

I don't know what it would take to get this done, and am somewhat curious in an academic way (hopefully the curiosity will stay academic).