determined,
Motorized bicycle riders in the state of California are required to have a M2 (motorized bike/moped) or M1 (motorcycle) license.
Go back one page in this thread and read what MotorBicycleRacing said in post number 2. He is spot on to what the vehicle code requires.
Also, the vehicle code here doesn't mention motor size, but, stipulates that the motor can not produce more than 2 horsepower (this is why the 138cc Whizzer motorbikes are legally considered mopes here).
I think the "under 49cc and no license required" myth came from a couple of sources. When the Chinese 2 stroke motors first started getting popular, that was what all the dealers claimed (under 49cc, no license required). It may have been true in other states, but, not here in California.
I also think some people got confused over a change in the law that pertained to scooters. About 10 years ago, they changed the laws so that you can ride a 49cc scooter (goped with a seat) without needing to register it. However, even with the 49cc scooter, the driver needs a regular class C license.
The only motorized thing that you can ride legally without a license in California is an electric bicycle.
If anyone tells you otherwise, they are either misinformed, or trying to sell you a bicycle motor kit......