While it's certainly possible that the infamous pin (Key #32 in the diagram) or the long pin that pushes on it (Key #34) in the 4010 PTO 540/1000 shift has failed, there are other things that can cause the PTO not to operate.
First, I'd have a look at the PTO "stub shaft" held on by 4 bolts. Are the bolts tight? If so, remove them, and check to see if any dirt or debris is trapped between the mounting surfaces.
Using a punch or other tool, press on the pin in the center of the tractor flange.
Is it spring-loaded, when you push it forward does it return rearward (as it should)? It is this movement that moves a shift collar towards the middle of the tractor and selects the 540 gear or the 1000 gear. (The 540 and 1000 stub shafts are different at the flange face, one pushes the pin forward, the other does not.)
Other things that could cascade the PTO not to engage...
When you move the PTO engagement lever forward, does it "snap" into the engaged position? If NOT adjustment may be needed, or something is wrong with the linkage, or the clutch, itself.
The PTO clutch is piggybacked on the engine flywheel with the foot clutch. There are a number of things that can go wrong there, including a failed throwout bearing, which can also destroy the pressure plate engagement fingers.
The lined plate can fail, including the hub tearing loose, or the splines shearing.
Take the inspect cover off under the clutch housing on note what (if anything) is laying on top of the cover.
Look up inside with a brite little light and check out the throwout bearing and the 3 PTO engagement fingers.
OBVIOUSLY, don't be mucking around in there with the engine running!)