Barnbikes
One thing you should be aware of is that the Corvair engine turns in the opposite direction of most other engines. If you just adapt it straight over without fixing this, you will have several reverse gears and only one forward.
Captainjunk touched on this when he mentioned the flipped VW transaxle.
If you ran an old Ford banjo type rearend, you could just flip the rearend over and everything should work OK. Should be strong enough for the typical Corvair engine.
Seems to me, I remember that someone used to sell a reverse rotation camshaft, too.
Makes for a very unique looking and cool setup, though. With a modern 5 speed and decent gears in the rearend, it should move a T really well and get good gas mileage, too.
Good luck!
Mike