just last weekend I received two trucks with stuck engines due to years of storage with fuel residue. The engines were completely stuck as were the carbs. I wish I had found this post earlier but my only solution was using a typical carburetor cleaner and spraying it through the exhaust and through the carburetor into the engine then letting it soak. Within the hour the engine was moving. The piston ring took a little more to free and required me to remove the glow plug and throw some cleaner in there.
These are fairly modern engines (Traxxas and Team associated) but they now run normally without issue. Based on what I am reading here, I could have damaged something, I guess I am lucky.

regards