If you remove the glow plug, and the engine turns over smoothly, then there's nothing blocking the piston. If you hear "clinking" sounds, that's normal when turning the engine over by hand. It's just the valve train working.

If you hear a clatter when the engine's running, it could be a result of an overheat, and you're hearing some piston slap at low throttle settings...you'd have to do a full teardown to check.

Your problems with inverted running are typical. You may have to lower the fuel tank a bit, add on-board glow heat at low throttle, or just accept a somewhat higher idle. Any of those could cure the difficulty.