I've got a 1979 Gilera 125 MX with rotary valve induction in my collection, it looks like a Maico with a straight axle fork. If I'm not mistaken MZ pioneered disk intake valve technology in the 50's and when key people defected to the west this design made it's way to Japan (and your Bighorn - gotta love the name!).
The Can Am's Rotax engine has an interesting history. Rotax began in Dresdan, Germany and was purchased by Fichtel and Sachs in 1932, whereupon production was transferred to Austria. In 1970 the company was sold to Bombardier of Canada and I've been told the rotary valved engines were already on the drawing board at this point.