Maybe the piston got stuck because the rings are bad?
Bad piston rings make an engine get sloppy loose, not locked up. In order of likelihood- - - -connecting rod bearing(s) most likely. Main bearing(s) next. Timing gears(a remote third place). Sticky valves or a valve seized up in a guide can lock up an engine, but you should be able to turn the crankshaft almost two turns in the opposite direction before the same valve locks things up again. Bottom line- - - -open it up and find the problem, then decide if you need to fix it or replace the engine.
Like the slogan Sun Electric Company used to put on all their electrical test equipment- - - - -"We test, not guess!"
Jerry
