I would blow some air into that cylinder to see if perhaps just a valve problem. That is after making sure there is still clearance at the valve adjusters on that cylinder. It does not matter if someone was at the valve adjustments some time ago you still have to check it.
A valve job is not all that bad if you know how to get things done. Or if not just doing the one bad valve itself is not a particularily good ideal but possible. It is not always actual bore/piston/ring problems.
The air test could also help establish if it was a head gasket leakage problem as well. Any air bubbles showing up in the radiator while running?