If you look at motorcycle's from the 1960's you'll notice there were more small displacement motorcycle's on the market. You even see mini bikes that most people would describe as being powered by a lawn mower engine. I'm not sure when the fad for mini bikes started, but I think the lawn mower powered mini bikes got pushed out of the market when more Japanese motorcycle's showed up. Most mini bike companies couldn't manufacture their own engine, let alone one that is a unit-construction design needed to survive in this market. Unit construction means the engine and transmission are housed in the same engine case. Here is an ad from Ducati in 1967 advertising their "Small Wonder" 100cc motorcycle engine used to power the Ducati Mountaineer, Cadet and Bio-Scooter.
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