On the left side of the tractor (sitting on the seat) there should be a silver tag on the clutch housing that has the serial number. IF that is gone, there is a machined pad under the number 1 spark plug on the block that also has a serial number stamped on it. Most of the time these numbers match.
Also you can check for casting codes that will be in the format of 10 17 X Where X would be the year. Post back what you find and we will help you out!