Sounds to me that your not getting any compression because your piston sieved because the rings were too big correct?? If I were you I would just buy a used 1982 nc50 head, jug, piston and rings, new top and bottom gaskets and sell that scooter. Get yourself something with a variable engine in it, Honda spree, SR50, elite, aero, hobbit, cameo and upgrade that to do 50 to 70 miles per hour. I feel like the automatic two speed transmission on the nc50 na50 don’t hold as well.
Contact VTCycles on eBay and ask him about different Japanese made big bore kits you can get for your express, they would probably know and have a lot of good information for you.
There could be a lot of reasons for now compression like a dirty combustion chamber, stuck valve, stuck valve springs, to big of a gap on a tappet screw, stuck ring, too big of a ring. Also I wanted to say one time my scooter wasn’t getting gas to flow properly because the hole on the gas cap was plugged up and wasn’t allowing the gas tank to breath, so gas would flow fast then slow all the way down. Goodluck