You related the engine was locked up, Were you able to detrermine why after the pistons were removed.My guess is that the cylinder walls are scared in some way shape or form so I would consider splitting the unit and overhauling the engine. Find a reliable machine shop that has experience with flat head engines. I have found with the cub engines I have worked on that the valve giudes have worn enough that will need to be replaced have them checked.You didn't mention the condition of the clutch or the transmission . so you may want to check the condition of them while you have the unit split. Steve