Of you look In the moped wiki all the information is under the Puch engines.
Regardless any 1.5 or 2 hp is worth getting. Like it was said already, only the 1hp za50 two speed is restricted via gearing, all the others are easy to mod for power.
Also, za50 bikes require work before riding. They have rubber shift dampeners that begin to disintegrate over time. Run a bike that's been sitting for 20 yrs or more and you risk snapping the countershaft when they shift.
Also, if you do get a za50 bike. Don't ruin the cover gasket when you open it up to change the dampeners. It's width is built into the transmission and replacing it can destroy the thing...
.....just thought I'd warn you of some traps that have screwed people over...look into the differences in the wiki and you'll be fine