Did you check to be sure the governor is working? With the engine running grab the governor rod and pull it toward the back of the tractor, you should feel the governor pull forward on the rod. If you feel the pull the governor is working. It is then most likely the carburetor is out of adjustment, dirty or as suggested the oil bath air cleaner. Try backing the high speed jet out 1/2 turn to see if it is any better. If so there's yer problem, the carb.
Here is how to adjust the linkages if you still feel that is your problem.
Have the backend of both rods loose. With the governor arm held back at its rear most position and the carburetor throttle shaft at its rear position (high idle) adjust the (short lower) rod so that it will just fit in place. Nest adjust the idle screw on the carburetor throttle shaft out (counter clockwise) until it is not touching the stop. Then hold the governor arm forward and have the throttle lever on the dash in the idle position, adjust the throttle rod (top ,with spring) to just fit in place. Done, remember to reajust the idle screw to idle at 450-500 rpm or so when running.