The attempt was in 1975, with the use of a 500cc snowmobile engine in a unique chassis. The rest of the bike was assembled using the TnT front end, cast wheels and a monocoque tank and seat assembly similar in design to Yamaha’s RD series.
Only a couple of prototypes were constructed, and were distinct for their unique exhaust system combining expansion pipes with a typical conical silencer to look like twin pipes. Unfortuneatly the effective ban on 2 strokes by the US Government due to emissions, cause the project to be scrapped.
I know of two examples still in existence. One in Alberta, Canada and the other in North Carolina, owned by Gary Robison himself, which was featured in the August 1989 issue of Classic Bike magazine. Link
The 1977 500cc CanAm street bike never made it into production. Only 2 or 3 prototypes were ever built (circa 1975). One was a street version which produced about 60 horsepower, while the other was an 85 horsepower road racing version. The engine was borrowed from a Bombardier snowmobile and modified to accept a 5-speed gearbox. The powerplant was liquid cooled and utilized a single horizontal rotary valve. The exhaust system was unique. The lower pipe on each side was an expansion chamber, and was connected to the top pipe (mufflers) with a small tube. This allowed expansion chamber performance while remaining quiet enough for street use. This machine never went into production because proposed EPA emission standards threatened its marketability in the United States. Tony Murphy wrote a magazine article about this motorcycle in 1976. He recently informed me that the prototypes are still running strong somewhere in Canada. Link
Some photos of a prototype spotted out on the road: