Laws vary state to state regarding motorcycle-moped-powered bicycle-scooter legislation.
Some states require a strict adherence to the powered bicycle/moped standard of pedal-start (no electrics) and require the bicycle to be pedal-powered (i.e. "assist motors").
All states that consider mopeds/powered cycles to not require registration, as far as I know, place a limit of 50cc on the powerplant. Our state, Connecticut, goes a step further and states a 30mph max speed and 2.0HP max output.
Here, mopeds and bicycles with engines cannot be used on limited access highways and some other designated routes. The grey areas come about from a couple things:
1) 50cc scooters built by major motorcycle companies and 1000 Chinese knockoff firms. Some are sold by motorcycle shops, some by anyone under the sun. Some are legal to the letter of our law and some are not, but broadly ignored if they are (or say they are) 50cc.
2) Powered bicycle (Whizzer) engine kits that exceed 50cc. Some engines are as large as 125cc. 66cc and 80cc engines are common and VERY hard to distinguish from the 50's. These go much faster, place much more stress on the bicycle frame and components, and are all technical violations of the law. Since they're also almost all "home installations", policing these is very tough. Small shops installing these and selling them to customers, knowing that they exceed 50cc, are putting themselves at great liability, I believe.
3) Other state's laws: CT does not provide for registering a 50cc bike or moped. No titles are produced, no insurance is required, no bill of sale is needed to sell one (though dealers collect sales tax as with any purchase). You can't register one here EVEN IF YOU WANT TO. Now, Rhode Island, due to a booming industry of rentals on an island offshore, requires ALL motorized bicycles, scooters, mopeds, whatever to be registered. So I can't bring my bike to Rhode Island legally, unless I somehow got a RI address and registered it there. Probably not worth the hassle. Similar complications arise when selling a bike across state lines, when one state has no paperwork and the other requires it. FIND OUT THE FACTS in your state before getting into trouble with one of these machines.
Yeah I still want to build one of those kits on one of my bikes! One place we've found that offers a nice turn-key setup is nearby in Massachusetts:
http://www.piratecycles1.com . They address some of their legal issues on their web site too.
I wish there were a single law across the US for these machines- presently its a patchwork.
--Rob