What could cause the crankshaft to not spin freely?

Suspect one of the exhaust valve seats has dropped out of the cylinder head, holding the valve open, and the piston is now hitting that valve.. *Can't be sure* but you can pull the spark plugs out and look into cylinder with a good flashlight for signs .. or the best way to tell is with a video scope or bore scope.. Look into cylinders 3 & 4 first.. Whether it worth fixing or not, I don't know.. You decide after checking it out.