It was in the late '80s that I saw that YS IIRC. Big roots type blower behind the backplate, similar to the supercharged OS four strokes. Makes sense that those belt drive Webra's had rotary valves. The head wasn't tall enough for there to be rockers up there...

The HB rotary valve four strokes had a shaft that drove the valve rotor. The shaft tube in the front reminded me of a bevel drive Ducati. I never saw one of those engines disassembled...

I don't remember seeing any OPS four strokes. Their aircraft engines weren't popular here for some reason. The only OPS engines I have any experience with are their 2 stroke car and boat engines. That square head .90 was a BEAST!