I saw an episode of Chasing Classic cars and the old guy "Roger" used Marvels Mystery oil on an engine / vehicle that had been sitting for many years. They removed the spark plugs and used MMO in the cylinders. After allowing it to soak for a few days, they placed the vehicle in gear and gently rocked the car, finally freeing up the engine. I enjoy when they show crusty old Roger and his tricks of the trade. The guy has been around a long time and appears to have knowledge about all kinds of vehicles. Perhaps investigate using MMO if you don't take off the heads etc. per other suggestions.