Building a street legal custom car is a pretty involving project. It's far more then just needing a windshield, blinkers, and brake lights. There are numerous requirements on the entire construction of the vehicle. If you don't base it off a factory chassis, then you've got an entire book full of regulations on building and assembly. It can be done, but it is by no means easy. Done right though, you could have yourself an AWESOME vehicle.
If I were to take on such an endeavor myself though, I wouldn't stop at just using a motorcycle engine. I'd look towards the concept Norton motorcycle tried using on their Nemisis motorcycle (which was never produced). They were going to take a pair of suzuki 750 heads and custom fab a bottom end to make a 1400cc V8 that would make 275hp. Now, something like that would be super sweet in a real small sports car chassis. I think the problem they couldn't overcome though was building a V8 crank in the dimensions of those 750 heads that could handle the power. There just wasn't enough room to make a beefy enough crank. Still, the idea has a considerable amount of appeal.
