King O Lawn was one company that used them on the aggies and some of the Mickey bikes too, there were others if I am not mistaken, but I cant think of them offhand. is it marked on the inside? Jeff clark was making recasts of them for awhile.
would make sense for an Aggie 98 swingframe (like in the add I posted above. They were 1969/70 and used the Briggs 5hp and had them mounted at an angle which looks like that one holes are drilled that way. are the mount brackets there in the box? They were usually just little L brackets that mounted to the engine sidecover perimeter bolts
I think the mystery is solved. My father in law (Don 81) said the mini bike was purchased at Montgomery Wards, it was named after a race car driver but he could not remember the driver's name. Well, that would be J.C. Agajanian or "Aggie" for short. Sadly the mini was destroyed when a kid on a motor cycle T-boned my young brother in law while trail riding. The Aggie frame was bent beyond repair and the only thing Don kept was the 5 horse Briggs and the clutch cover. I'm currently rebuilding the 5 horse for another mini bike project.