The newer 84 12A is going to be a lot easier to find parts for. The older 12As had different seals, ports were a lot smaller, plates weren't nitrided, and the twin dizzy ignition was more temperamental. Just to mention a few things.
Why do you want to use the original engine? I'd just store it, in case someone wants to restore the car to original sometime. Although I can't forsee them becoming a highly sought after collector car anytime soon. Most of the people that want them would rather have them driveable.
The Factory service manual and wiring diagrams can be found at:
http://foxed.ca/index.php?page=rx7manual
You'll need to review the factory wiring diagrams for the RX2 and the 84 RX7 to see where the differences are at. These are really good diagrams, so it's not like trying to make sense of those Haynes manual pieces of
.
Once you've figured these out, you'll be able to see where you want to make your changes. The newer electronic ignition is a lot easier to deal with than the twin dizzy. You'll also need to wire some of the other functions of the 84 carburetor (power valve, Air Vent solenoid, & coast richer solenoid to a 12V Ign source, see manuals). You can avoid this if you use the original carb/intake from what I hear.
If nothing else you'll learn a lot about the car in the process.