I built my own bicycle with a 4 stroke 49cc motor on it. as I was afraid I wasn't complying with the California laws when riding it, I stopped with my motorized bicycle, and talked with police officers on 2 different occasions.
I showed them my motorized bicycle and asked them if there was anything I needed to be able to operate it on the streets legally. both times the officers told me no. I didn't need any type of registration or license.
In addition to those times, I have passed police that are in their cars many times and wave to them as I go past them. I have never been stopped or pulled over by one, they just smile as i go by.
I have been riding my motorized bicycle for quite a while now. I have logged over 1,000 miles on it riding it on the street. since I live in Orange County, when I ride, there are times when I hit some pretty heavy congestion. I also have to change lanes in order to stay out of the lanes entering and exiting the freeway. I use my hand to signal the other drivers who have always been courteous of me and treat me like another vehicle on the street.
I enjoy riding it so much that I ride it to and from work which takes me into the wee hours of the morning. I have also passed police at 3am in the morning and have not had a single problem with them.
As I go through 4 different cities on my way home and have encountered police in all those cities, I can safely say that I am not required to do anything other than ride my motorized bicycle. so I dont know why people are saying motorized bicycles need to have a special plate and or license. I have attached a picture of my motorized bicycle so you can see what I have been talking about.