The revs in the ad were quoted as being in the high 3,000 to low 4,000 for cruise settings. Isn't this similar to the Rotax 4-strokes? Scooby's have a reputation for pretty bullet proof engines in their cars where a lot of them spend a large part of their time in upper reaches of the rev range ie; 'normal' cruise rpm is around 4,000 rpm @ 100 mph.
I don't know, but think it could it be safe to assume that there have been a few less than professional conversions of the Scooby flat 4. How many were lifted from scrapyards rather than converted from factory new engines, I have read the articles written by people who have changed the oil and determined from the cleanliness that the engine was OK. Anybody who takes such a cavalier attitude to their engine are asking for a failure IMHO.
As a further comment, gearbox design can have a significant effect on engine reliability due to the vibration and bending loads transmitted back to the crankshaft from the prop / gearbox combination. Isn't Rotax up to the C model of gearbox in a relatively short space of time?
Adapting a gearbox to suit an engine may give you the speed reduction you're looking for but it may transmit unacceptable vibrations and loads which in the medium to long term will wreck your engine.