Renegade Hybrids has a good reputation, they used to do a V8 kit for the 924, although they stopped selling them because it required a complete drivetrain swap from a 944, and that was just too expensive for 924 owners.
I considered the V8 route for a 914, but I came to the conclusion that I would rather emphasize handling and braking over straight line speed, and that a 3.6 liter 964 or 993 engine massaged to around 300hp would be a better choice for my application, although considerably more expensive, at around $6K. My reservations about swapping watercooled engines in is that they are generally heavier than the air cooled units Porsche uses, and you have to add a radiator, coolant and all of the plumbing, which adds a lot of weight.
A smallblock Chevy is typically 450-600lbs, even an aluminum radiator weighs 20-30lbs, 20lbs of coolant to keep it cooled, another 10-20lbs for the plumbing to get the water to the front of the car, vs a 300lb engine that cools itself.