That is a 120SF. The short snout between the pushrods and the thrust washer, lack of a horizontal rib between the big part of the case and the front bearing housing plus the big thrust washer The valves may have been replaced with a newer version at some point which would explain why the keepers are different. Also the exhaust port thread is the same from the YS 91 - 1.75 so any muffler or pipe could be used there.