Put a few squirts of gear lube in the cylinder, turn it over a few times with the plug out, then check it. If it goes up, it's the rings. If not, probably a valve. May only need the valve clearance adjusted. Let us know what you find.
It may have broke a ring. One of my Honda 125's did. Did you just take off the head or pull the jug, too? With the head off and cold, you should show a few thou clearance on the valves. They should be set with the piston on TDC on the compression stroke and the engine warm. Where to find rings? I don't know. Just found the supplier for Loncin parts the other day after a couple of weeks of serious searching. If it's a GY6 clone, parts are out there.