How sure ?-----I am not there with hands / ears / eyes on your motor.-----In my yard I would just go to one of a few parts bins to put another one on.
Look on the top of the cylinder block for a round quarter-sized disc with model & serial numbers stamped on it. It may be hidden by the electricals bracket on yours. Look for numbers 135*****, where 135 is the horsepower, and the next number is the year. "3" = 1973, "6" = 1976, etc.