The only thing I have ever done to mine, which I inherited from my father was to change the oil and replace heads/valves. There was an early and late model version of the valves. Early versions would break, newer ones last much longer, the spring is the failure part. I have been running one of...
I have two of this version and one that uses a different casting for the block. Assured Air in Douglas Georgia was keeping heads and gaskets in stock but I think they went out of business in late 2024 early 2025. 912-384-9254
Here is the NAPA diagram and parts list I have.
Head gasket pair...