I used to have one, but eventually traded it away without ever messing with it.
Seems to me they are mentioned in Wendell's Gas Engine book.
Here's a quote, found on another site, that gives SOME details:
"I found reference to the "Busy Bee" engine in Alan C. King's - Gasoline Engines 1909 - 1962 on page 24. I believe it is a reprint of an ad from 1945, that includes a drawing of the engine. The ad states that a revolutionary 5 HP all-purpose engine has been anounced by Kinner Motors, Inc. of Glendale, CA, engine manufacturer for the past 26 years. Called the "Busy Bee" the Kinner model AB-3 is an air cooled, 4 cycle, L head engine rated at 5 HP @ 2,600 RPM but at 6 HP @ 3,250 RPM. Bore is 2 3/4" with 3" stroke, displacement 17.8 cubic inches. The cylinder is horizontal and detachable, making maintenance easier. Full pressure lubrication is supplied by a positive high pressure pump. There are other features mentioned in the ad. It's possible Kinner Motors was taken over by Gladden Products Corp. as both show the same address."