Re: the problem with motorbicycling
Not sure exactly when it was, but here in Mass, they recently added an option to the moped registration form covering these bikes. Instead of checking the "New" or "Used", you check the "Converted bicycle" box. Then you fill out and sign an extra form called "BICYCLE CONVERSION to MOTORIZED BICYCLE (MOPED) AFFIDAVIT of OWNER". You have to provide the details for the bike and serial number of the engine, and attest that the engine meets the requirements of any other moped. Then you can pay your $40 and get your moped sticker.
So if you install the centrifugal clutch, and tell them the engine is only 49cc and goes a max of 30mph, you meet the moped requirements and you'll be legal. You then have the right to ride on all public roads, except limited access and state highways where there are signs prohibiting bicycles.
So I'm glad they updated the laws to allow these bikes, rather than trying to ban them.