ok, if you want a stock moped that does 45mph or more ... it all depends on what you mean by 'stock' and/or moped. most older mopeds did around 30mph tops at 'stock'. but stock is a relative term. my bianchi was built to go 15mph (not joking!) but it now runs at over 40mph. most of it is still 'stock' in a certain sense (at least the parts that got it to about 30mph.
the reason most people aren't giving you the kind of answer you're looking for is that few mopeds do more than 35mph 'stock' in any sense at all. those that might, are hardly ever imported into the us.
unlike some newer scooters, mopeds didn't have restrictor plates, it's just a matter of specific parts. my bianchi did 15mph 'stock' because it had a small carburetor and slightly less-than-50cc cylinder/piston. so upgrading it to a regular carb and piston/cylinder brought it up to about 30mph (which is 'stock' in other mopeds w/ the same engine).
now some older and newer mopeds do 30-35mph 'stock' quite easily. there are a few motobecane mobylettes in town that do about that much and are pretty 'stock'. a vespa grande will do 30-35 'stock' as well. i think some garellis are also fast, so are some sachs. with a little tinkering, most mopeds can do more than that.
now, i do hear that some new mopeds (esp. the derbi) do about 45-50mph 'stock'. so i'm sure you can find a moped that goes that fast. but ... it also comes down to availability. so you might have to go w/ a slightly slower moped and mod it out.
now a new derbi can cost as much as $2k (if you can find one in the us). that's a bit of money. for $300 or so you can get a good moped and spend another $200 or more to mod it out for speed. it might be worth it to you. and it's not hard ... the total work hours to mod my bianchi to do 40+ was about 45 mins not including the time it took me to get it to a 'normal' 30mph. that's how long it took me to pop on a new carb, polini exhaust, and larger front sprocket.
many states do have speed limits on mopeds. but i'm not sure they're enforced. michigan has a 30mph speed limit on mopeds, but i've never been pulled over on my bianchi.
now scooters can go a bit faster, but most are also restricted to comply w/ state/local speed laws. i don't know much about scooters to know how to change that, though i've been told it's also rather simple.
the best solution ... if you have the means ... is to find a fast moped/scooter out-of-state (if your state has speed restrictions) and then just bring it into your state. our experience suggests that most dmv workers are rather ignorant of moped/scooter laws and regulations ... so just go up w/ your bill of sale and register the thing as a moped.
not sure what else to tell ya. good luck.