Finally contacted Kohler and spoke to a fella in the tech dept. Described what the original user told me about the failure and what I'd found, or not found. He said since it happened suddenly, and there is no scoring of cylinder wall, and no blown head gasket there was one more tiny thing that might cause it. On the cylinder head is a small reed valve that may have a piece of carbon or other debris, blocking the reed valve from closing. No guarantees, but worth checking. Welp, it's running fine now. Head scratching is over for now. Thanks.
