The Yamaha QT50 or Yamahopper is a noped (moped style frame architecture but lacking pedals) produced by Yamaha in from the late 70s to the late 80s.
The most outstanding feature of this bike would have to be it's final drive which consists of a solid drive shaft (with a torsion spring to dampen the engines vibration) that connects the transmission to a ring and pinion gear set in the rear hub. This setup requires no maintenance and has the safety feature of lacking any exposed moving parts such as a chain that could catch one's clothing. The bikes all came with turn signals and oil injection making them very modern compared to many of their European counterparts. All but the very early years of production had CDI for ignition.
These bikes like most Japanese bikes of the time came with excellent finishes in both the paint and chrome, though the plastic parts on many of these have deteriorated over the years. The engine is smooth running, reliable, and long lived when proper maintenance is observed. The wheels are sturdy for being only 12" but abuse should be avoided.
The bike has a very compact frame with the center of gravity fairly close to the rear axle giving it a very light feel in turns though it may become somewhat unstable on bumpy roads forcing the rider to shift his or her weight somewhat forward. The rear brakes tend to be weak on this bike.
The QT50 drive train was shared by the Tri-Zinger and PW50. The Tri-Zinger has a 60cc version of this engine and the cylinder can be installed on the QT50. The PW50 is a children's dirt bike which has different gearing in the hub, which can be used on the QT50, there are also performance parts made for the PW50, many of which will fit the QT50 though parts that must attach to the frame may need to be modified.
Sharing an entire drive train with a fairly current children's dirt bike means most things can be had readily from your local Yamaha dealer. Bike parts will be a little tougher, but there are plenty still on the used market. Plastic parts may be the most difficult of all as they did not age well if left in the elements.