Motorcycle engines are designed from the ground up for moving about 1,000 lbs weight passenger and vehicle. If you add more weight and create more drag, like that of a car or truck, the fuel economy will drop as well as create a lot more engine wear. By installing a motorcycle engine into a car you would only be causing a lot of strain on the engine and the fuel economy would drop considerably. The power would be quite poor when you calculate the power to weight ratio in contrast to that of a regular car.
Personally I think you would put a lot of work into a slow car that would wear out fast and would probably not have that great of fuel economy.