There is something much more fundamentally wrong with your bike in my opinion. All 50cc two strokes should be able to do around 30-35mph no problem. True, the engines are restricted, but are also designed very specifically to do this range of speeds.
Mopeds in general are inexpensive products (compared to a motorcycle, etc), and are not "performance" machines. This means the manufacturer can get away with making less precise, poorer quality parts. Therefore, most likely, things inside your engine are "out of spec." Hell, even expensive performance two-stroke bikes have these problems. I would recommend taking the engine apart and blue-printing it. Meaning make sure the engine is to design spec. I know you don't have "design specs" (well, if you do kudos) but all you need to do is match up the intake manifold to the cylinder or crankcase (depending on piston ported or rotary), smooth out all the transfer ports and boost ports (if you have one), and match these to the crankcase also. Just mark one side of a mating part with a non permanent magic marker, reassemble the parts, and then look at where the overlap lies (now marked on the opposite part in magic marker). Just line 'em up now.
Worst case you don't have to do this but you could go through all the ports and clean them up with varying grits of sandpaper. I've found various two-stroke engines that are so mismatched it's rediculous, or the ports are so rough the engine runs o.k. all the time, even when new. Fluid dynamics are so very important in two-strokes that smoothing out everything just makes it easier for your engine to do it's work.
Of course, there are a ton of other problems that could be wrong like worn rings, clogged exhaust, etc. Sounds like you have checked all this? As usual, the How To Fix Your Ped should be referenced here right? Hehe! But cleaning up manufacturing flaws is a great way to get you bike tip top without really messing with dangerous modifications. Sorry if that was too much garble, but hope it helps a little.
> Look, we got Isaac freakin Newton up in here! :) Just
> kidding. Jamie, thanks for the thorough explanation. Now I
> uderstand completely. You are the man!!