Harry, if your moped ever suddenly stopped while you were riding it then you most likely had a soft seize and that is why you have lousy acceleration.
This happened to me so I bought the Airsal 70cc engine.
When I took out the old piston it had 2 smear marks on the top side - intake side; not bottom exhaust side.
If you plan on making a purchase, I recommend you purchase a complete kit with carb, exhaust, and piston/cylinder.
Good luck and "Ride riders ride!"