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.