Does the PTO lever move when you press the clutch in? Does it move when you power the implement? If so the bearings in the transmission may be worn -- you don't want to think about that.
There is a sliding coupler that connects between the transmission PTO output shaft to the actual PTO shaft. It could be worn or not getting coupled to the full extent.
There is also a common issue with the PTO shaft bearing getting worn after 50ish years. If you go the back of the tractor (engine off), grab the pto shaft and push/pull it forward and back --> does it move? If so, the bearing is shot. Easy to fix. Park nose down a steep hill, take out the 4 bolts, then pull the pto shaft out. You may have to yank it hard to break the gasket seal. Press off the old bearing and press on a new one. Or if the splines on the shaft are buggered up, a new assembly (new shaft and bearing) is about $150.