I opened the engine and saw that it's gummed up pretty bad, so I'm going to try soaking it in mineral spirits for a while.

Is it normal for the piston to be caught at TDC while idling after running? When I see that there is almost no fuel left, I usually just let it idle until it stalls and then put it on the starter box with the glow plug to burn off excess fuel, but I couldn't do it last time.