Exhaust valves.
I had a 2007 S-LSA with a factory-new O-200-A. The engine was a selling point for me at the time. I've been flying airplanes (mostly C-150s) with the O-200-A since the mid-1960s, so I figured I knew how to operate it.
It needed new exhaust valves on all cylinders at 125 hours TT.
I was told Conti had recently redesigned the exhaust valve seats. A significant number of that make/model S-LSA had the same issue. Those that had the cylinders reworked by reputable outside shops had no further problems.