Maule M5 pilot reported engine malfunction in cruise flight. Post-flight maintenance inspection revealed exhaust leaks and cylinder damage.

Narrative

In flight; engine suddenly began running rough; about 25 minutes into a flight from ZZZ to ZZZ1. I was at 7500 MSL just south of ZZZ2 and turned that direction while sorting the problem. Approach was notified. I descended to 6000 MSL while adjusting the ignition and fuel mixture to get somewhat smooth operation. The EGT (Exhaust Gas Temperature) indicated a bad spark plug on Cylinder #4. At partial throttle; the engine was smooth enough to continue on to ZZZ1; where I would have access to my maintenance hangar and tools. I climbed to 8500 MSL keeping within glide distance of an airport until arriving at ZZZ1.After safe landing; the cowl was removed. It was immediately discovered that the right side exhaust manifold had two of three support tabs broken off due to bad welds with very poor penetration. This allowed an exhaust leak on cylinders 2;4;6. The exhaust was repaired and re-installed. Very low compression on Cylinder 4; with the air blowing into the crankcase. Borescope showed a few dings in the piston crown and severe erosion of the piston corners. The valves were not damaged. Later disassembly showed that the top piston ring was broken and clamped into the ring groove. The second ring was also broken. The oil control ring was intact. The upper spark plug on Cylinder 4 was missing its center electrode and about half of the ceramic insulator. The electrode was subsequently found in the exhaust pipe baffles. All new spark plugs had about 20 hours of operation. Further inspection of the cylinder showed that it was 0.010' out of round.