XS650 enthusiasts will be surprised to learn that the most exotic XS650 race bike ever built was created not in the US but in Japan, by Japan's gamble racers (auto race). This is state-sponsored gambling on oval track motorcycle racing. The heydays for gamble racing were the sixties and seventies, when top riders made serious money, a fact reflected in the tuning of their race bikes. The favored engines in those days were non-unit BSA, Triumph, and Meguro (a Japanese copy of the Triumph but with some modifications/improvements).
Anyway, my buddy, now retired, was one of those guys and he showed me a photo of an experimental auto racer they built using the XS engine, and it was wild! They cut the whole gearbox/ wet clutch off the the thing (along with all the cooling fins on the head), drove the oil pump off the right end of the cam and put a primary drive sprocket on the left end of the crank, driving a typical dry clutch/non-unit type gearbox.
Only the one bike was built and apparently it didn't offer any better performance than the other motors, so only the one bike was built. I've got a photo of it somewhere and will try to find it.
Eventually, the old twins were superseded by the HKS650, a very powerful Japanese racing engine, which was in turn replaced by the very boring Suzuki race engines they run today and on which no tuning is allowed.