why would you want a rear wheel drive gear? - this car is FWD but with a longitudinal gearbox like a subaru boxer or VW boxer. If you are trying to convert this into rear wheel drive then its a far cheaper suggestion to scrap the car, the amount you will spend on it would be collosal and the result would be absolutely terrible.
I have seen one of these converted to RWD in Karachi and it was a hopeless car after that. The owner actually left it to rot outside his house.
If your reverse gear is not engaging, have you made sure the selector linkage bushings are in good shape and aligned correctly? - Are you able to select all gears with engine off?
If you are thinking of making it a transverse engine car then it also is a HUGE pain as the body structure is very narrow in this car - it was built to take a longitudnal mount engine with 90 degree driven gearbox - thats why the engine is shoved all the way to the front of the car and the radiator is on one side of it because there was no other way to mount it.