I think I would look at pulling the head and hope that the head gasket is bad. If not you are probably looking a t a valve job to start. But as always, you clean up the val;ves, then there is more compression and the rings go. So where do you stop? I think at least pulling hte head gasket is a start. Then look at is and he head, see of there is leakage between 4&5, or from the cylender to the outside of the block. There may be carbon buildup, or some damage. Then be sure the head is flat. It is at least worth pulling hte valves and seeing what condition they are in.

Ted Wilson