Sorry Bill no experience with that bike, I just checked my reference library and found this Japanese bike with pedals, then checked under photos on this site and found the pics.
There were probably different models as with most mopeds, 20, 25 and 30 mph models according to the state where they were originally sold. My refernce library says 30 mph was top rated speed.
You might try removing the muffler and take it for a test run and see if the speed improves. If speed does improve, you probably need to decarb the muffler, exhaust port, cylinder head and piston. Also put in a new spark plug with the proper gap. See if your local Yamaha dealer can help you with this info.
I don't know if it is a belt, chain or drive shaft driven. If it is belt driven. a new belt would probably help or checking the tension on the belt. If it is a chain drive, clean and oil the chain and make sure it has the proper tension.
This bike is oil injected so make sure your oil lines are open and the oil pump is working or you could ruin your engine. If in doubt you can plug the oil lines and run premix in the tank.
This is also a pretty heavy moped (168 lbs). The weight of the bike plus the weight of the rider has an affect on top end speed.