I don't see a piston seizing at idle, which is when it stopped. A piston seizing from heat is almost always at a high power setting. Sounds more like some foreign material in a cylinder. Or a valve seat dropped.
I'd first check that all valves are free to move and also check valve lash- see if any have changed much from normal settings.
Also seen where a cam drive gear got loose on a camshaft, caused valve timing to drift and cause all sorts of strange issues. Pistons were hitting valves.
And also seen a drive plate spring come loose and jam the flywheel.
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