I have built a couple of mopeds (actual pedal mopeds) that would exceed 50 mph, legally they are limited to 30 mph. You just have to be careful where you ride them. Neither one of those engines had a very long lifespan, the power and high RPM simply overwhelmed the stock bottom end.
I'd say a Zuma 2 stroke is the way to go. The Genuine Rattler 110 2 stroke is interesting, but I have no idea what is available for it. You could get away with a Zuma, if it has a motorcycle plate on it. No displacement or speed limits on motorcycles. In my state everything BUT pedal mopeds MUST have a motorcycle plate on it, including 50cc.
The Ruckus with a GY6 swap is a whole nother matter. It is about twice as long as a stock Ruckus, and would be real easy to spot. I'm not sure what those guys do. Even in states where a 50cc has to have a motorcycle license and plate, what do they insure them as? The VIN shows 50cc, but a GY6 is 150cc. If you insured it as a 50cc, your insurance would not likely be any good. In my state it could be registered as a special construction vehicle, but that is hard to do.
2002 Vulcan 750 (engine out, slowly being disassembled) 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
2012 Zuma 125, 1980 Puch moped