Porsche G50 may be the strongest unit, but it will also probably come the most dear from a cost standpoint and that's only the transaxle. Then you have to begin engineering and buying all the other costly stuff to make it work.
Rancho High Performance Transaxles and Transmissions | RanchoTransaxles.com
I've always been a cheap SOB, so have figured out how to do stuff differently and inexpensively most of the time. I was just thinking, what if a guy used a complete FWD engine/trans/diff/suspension unit from a late model vehicle and placed it amidships in the Vega? From the late 90's until the end of production in 2004, Cadillac produced the Seville with a 4.6 liter, DOHC V8 that spec'd out at about 275 horsepower and 300 ft/lbs of torque and 4-speed auto. That wouldn't make a killer acceleration vehicle out of the Vega, but corner-carving road-handling (with some aftermarket help on the front suspension) and fuel economy ought to be outa sight and it should be a blast to drive. The '98 to '03 STS model was rated at 300 hp. I'd put the car on a lift and make a cradle around the powertrain/suspension before I excised it from the car.
Cadillac Seville - Wikipedia, the free encyclopedia
I might begin by searching for a complete car that has been crunched in the rear or side and use everything out of it, including all the wiring and electronics.
Once you find a donor, get all the factory manuals for it so you aren't flyin' blind......