From your engine's model number it sounds as if it has the reciprocating type of Synchrobalance, not the counter-rotating shafts type. The reciprocating weights seem a bit prone to wearing out if the oil is dirty. Hopefully your engine has always had clean oil, and has just been dusted a bit, which may not be terminal with that cast iron bore. You'll probably need new rings, and you can hope the piston has survived: you'll find out for sure when you do the rings. It's mainly because of the Synchrobalance that I was concerned about the engine's history. I wouldn't be worried about the owner having pre-warmed it, given that it seems to be running very lean: you can fix the mixture in a couple of minutes. You might check how clean the fuel tank is though, in case there is some dirt in the fuel system - something has to have changed, to make the carburetor run lean, and a small amount of dirt is one possibility.
