Why would you have to reverse the final drive? You're not turning the engine totally around are you? That wouldnt really work. The engine still faces forward but moves to the back of the car. Been done many a time, and is how the Fiero, MR2, etc are setup. Its Mid-Engine, RWD.
Unless you go to a true Rear-engine style transaxle like the old VW bug or Porsche, where the engine sticks straight back out of the car. That can be done with adapter plates, but I think everything would be too long then.
The Miata has *totally* different ECU & harness. It uses a CAS & coilpacks for ignition instead of a distributor.
As for making a Miata a 3-wheeler: I dont think its possible, really. The engine sits in the center of the engine bay (setup like a traditional RWD car), and low, there is no room for a wheel in the front.