I've got a VR6 powered Jetta. It's a great engine, I bought my '95 Jetta new and have put 163,000 absolutely trouble free miles on it so far, the only thing I've done to the engine so far has been oil changes, two sets of plugs, one set of pllug wires, a thermostat, a belt tensioner, and a water pump. The engine is still in perfect shape, and it still pulls like a train and makes the most pleasing howl out the tailpipe.

Dimensionally, it's scarcely bigger than a B20 - not any longer and rectangular in side profile, and only a couple of inches wider. It's more of a shortened I6 than a narrow V6 really - a single head bolts onto the top. It also doesn't use a high mounted torque are either, it mounts similar to a B20 in the engine bay - two mounts on the side of the engine, one on the transmission. You;d just have to find a suitable/fitting RWD style bellhousing/transmission.

Also of interest is the VW TDi (turbo direct injection diesel) engine. 90 hp, 150 ft lbs stock, and they get 45 - 50 mpg in a heavier (than a PV anyhow) Jetta body.

Another engine/transmission that would be more sports car than muscle car would be the Honda S2000. It comes from a RWD car so it's already a neat package ready to fit into the car. The engine is racier than most full on race engines - with a 9000 rpm redline and 240 hp from 2 liters in stock form, although torque isn't as easily fooled by high rpm technology, at 150 ft lbs. The transmission is a slick shifting 6 speed manual. These packages, pulled complete from wrecked cars (apparently a generation of FWD racer boys can't always graduate into a RWD roadster without a few 'incidents') complete with electronics, sell for about $5 - $8K. I suspect one of these would work very well in a light PV, although I'd be hard pressed not tryong to find a tired old Lotus Super 7 if I got my hands on one.