This happened to me last weekend. I was driving on a hill and after a stop I was able to ride the moped only couple hundreds meters and an engine got stuck. After waiting over an hour I was able to start the engine again.
Today I started the moped and it did sound odd. I checked the cylinder and there were small scratches on both sides of cylinder. I didn't even bothered to take a look a piston.
Were I could find a new piston and cylinder for my moped?
I've been using 3% mixture on it. Is it possible that a carb is tuned off? Maybe too much air? The carb and the needle are original ones.