This article suggests that they started with a marine engine, typically an automotive engine that's been modified for these duty cycles by changing cams, timing, bearings, etc. Probably not all existing automotive engines are well suited to these applications, but they only need one or two.

Obviously an aviation application has other added constraints like elevation changes, inverted operation, packaging requirements, etc, but "runs at 75% max power for hours at a time" is certainly in common with the marine applications, and trivially demonstrated on a dyno stand.

GA does not fly inverted; elevation changes are influencing mostly the air mixture. Just saying, not contradicting.

Old marine applications would use carburetors and often those carburetors weren't at all suited to changes in elevation. (Most of my knowledge is actually only of a specific family of Volvo / Penta pushrod engines)

Probably airplanes, even in GA situations, are able to operate under negative Gs far more often that a typical marine application.... A person adapting such an engine would ideally think about oil pickup under such scenarios.