Regarding FIAT motors and transaxles in general:
The 124 series run clockwise, so a reversing gear is required for a fwd transaxle to be used in a rear engine application in addition the differential needs flipping. With minor grinding, this can be done in a Subaru transaxle.
Subarugears in Australia sells reversing ring and pinion gears.
The 600/850 series run counter-clockwise, so NO reversing gear is required for a front wheel drive transaxle (from a fwd engine that runs CW) used in a rear engine application. But, the ring & pinion will run on the back side (or coast side).
So with 600/850 blocks you can conceptually use any transaxle from a FWD CW engine: Renaults, Citroens, Alfas, Subarus etc., but they will all load the back side of the pinion gear.
Running on the coast side could be a problem, as pinion gears are not designed for that. The solution, or trick, is to purchase a reversed ring and pinion, but NOT flip the differential. That way the power goes through the proper side of the gearing. At least that is the way I understand it.
Again, Subarugears sells reversed R&P's.
Subaru transmissions are strong like the Renault transaxles. Most are designed for 300HP and at least 225ft-lb of torque.
The only missing link is the engine/transmission adapters..but these can be custom made.
More info here:
http://www.thesamba.com/vw/forum/viewtopic.php?t=503586 [nofollow]