There's one other possible cause if it's not rust from being put away with raw fuel in it. If the conrod is stretching it may have allowed the piston to hit the glow head. If that happens it quickly mushrooms the top of the piston and the result is that it jams in the bore.

Either is not a nice condition to have but of the two I think I'd rather it was just some varnish buildup from the fuel or even a bit of rust.