in most states, to be highway legal, it must have a license plate and adhere to all motorized vehicle requirements (lights, safety devices, must wear helmets).
For motorcycles or scooters over 35mph (in most states, some are 25mph), you have to register the vehicle and have it insured. If I limited my motorcycle to 35mph, i could register it as a scooter, because legally it fits the definition of a scooter (automatic transmission or one gear, 35mph limit, all lights, no highway riding).
for a scooter to be converted, just make sure it adheres to the definition of scooter. If you make it faster, realize you can get a ticket, or you'll need to register it as a highway capable scooter. Getting the title changed isn't as much of a problem as it is with insurance.... start with insurance. DMV just changes the fuel type to electric....