If the pistons froze from corrosion, you may be able to pull them, slap some new rings on it and just do a hone on the cylinders. I would probably do a new cam chain, at the least new sliders. You may or may not need to have the head decked. But, it could be much worse. No telling till ya get into it. If the marvels frees it up, I would say you are in good shape. Many people on the board are riding bikes that were freed up this way and they never even pulled the heads.