Unless you plan on going into the power generation business your generator is never going to operating at the 50HP level. I have a 12kW standby generator with a 30hp V-twin engine and I've never even come close to taxing it. If you just remeber to limit yourself to two 240V appliances at the same time (which IMHO is not all that much of a restriction) 10-12KW will be just fine for standby power.

I like the idea of keeping the engine/transmission in the car and mounting the generator head in the trunk. If the cruise control works you could also it as a generator load governor!

All in one neat package that you could push around with an old garden tractor.