what is legal to drive on street
When you hear the term, motorized scooter, that's often what people are referring to. But the term street legal scooter is more often used to mean the same thing as a gas scooter or motor scooter. However, you may find that on this site I often interchange the terms street legal scooter, motorized scooter and gasoline scooter.
Depends where you are. In the UK, you can buy a street legal car which looks (quite a lot) like an F1 car. Furore Formula One Kitcar. Obvious additions are mudguards, headlights, speedometer.