I'm thinking a mouse nest under the engine shrouding. Those meece just love to build their nests in there, and they even have an area to use as their toilet! If that has happened, the cooling air can not get down around and through the cooling fins on the cylinder and block, which then causes excessive heat build-up inside the engine, and that causes the engine to bind up, and cause more heat. Left go too long, and you'll be looking for an engine!
Brfore buying a new magneto, or carburetor--check under the engine shrouds. Might get a surprise! I opened one up and found mother mouse in the last throes of child-birth! She and her kids didn't live long enough to become a problem!! "STOMP!"
And, next fall, when you put it in storage, stuff a couple clothes dryer sheets in around the engine shrouds! Mice hate the smell, women love it!