The Z50 is a hard-to-find 2-speed manual engine made by Puch.
Although it fits into the same engine mounts as the E50 and the ZA50, the Z50 was never used on any American bikes, due to moped regulations requiring the use of automatic transmissions. The Z50 utilized a hand-shifter on the left grip, replacing the left brake lever with a clutch lever. Any bike equipped with the Z50 engine had to use a foot-pedal engaged rear brake for this reason. They are extremely scarce in the United States, but a decent amount have made their way over. They are much more common in europe, unlike the ZA50, which is very rare there.
The key performance advantage to the Z50 engine is the lack of a centrifugal clutch, which is hard to control and will wear out much quicker if engaged at high RPMs. Also, having 2 speeds to choose from allows for taller gearing while retaining decent low end.