You can see the pin that runs in the PTO clutch groove, through the transmission oil filler plug hole, using a light.
Below is a listing for a new lever and shaft at TM Tractor, the short pin on the bottom of the part runs in the PTO clutch groove. They are originally round. They can wear and become flat, then it has too much play in the PTO clutch groove.
http://www.tmtractor.com/new/pt/317fp.htm
If you don't do repairs like that yourself, you would need to find someone that knows how to repair it.
You could loosen the belt for the mower, and see if the PTO will engage.
Below is the Cub service manual, the PTO is in section 6A. It explains how to remove it.
To replace the PTO pilot bushing, you need to remove the rear cover also, to get to the bushing.
If you don't have the Belt Pulley assembly shown in the pics, disregard the info for it.
http://farmallcub.com/rudi_cub/www.clea ... index.html
Sometimes the front of the PTO shaft, that runs in the PTO pilot bushing, is worn, or rough, when the bushing is wornout. It may need a new PTO shaft also. TM Tractor, at the bottom of the page has new shafts.
Below is a pic from TM Tractor of a Cub trans housing with most of the parts removed.
The PTO pilot bushing is where the bearing in the upper hole is.
