Morris Marina in the workshop

The Morris Marina that comes in every once in a while was dropped off yesterday for some investigation. It had a leaking carburettor jet when it was last in so I rebuilt both carburettors but seemingly since then the engine has been running unevenly. When I drove it into the workshop last night it felt like it was down a cylinder so the first thing I did this morning was a compression test, that showed cylinders 1 & 2 were ok at 150lb but 3 only had 50lb and 4 barely scraped up to 100. That would explain the rough running. I whipped the head off, kind of hoping to find a burnt valve for the sake of the owner's pocket, but all looked ok at the top end. A peer into the bores provided more of an answer, the markings on the 3rd cylinder indicated a broken top compression ring and poking a 0.020" feeler gauge between the piston and the cylinder, as well as showing a lot of cylinder wear, confirmed it. There's no alternative but to remove and rebuild the engine. Oh dear.