Yes, you can rebuild it on your own, but I doubt it is needed if it still worked. Clutch pack maybe, but the rest of the pto should be good if it was still running the machine. The pump is just a small hydraulic pump inside the housing that engages the clutches. It has a prv that can be adjusted a small amount if needed to attain the pressure and flow required if the tests show they are off.

 Usually if the flow rate is above the specs, it may indicate a worn piston seal pack. It can still ahve the rated pressure, but flow high. If the flow is low, likely the pressure will be also which may indicate a worn pump or faulty prv. Anyhow, since you have the manual, you are better off than 90% of people. It will give you reasons for any out of spec readings.

 I know it is frustrating. Buy a machine and hope you are a fair judge of character, then find out the machine feels like money pit. But dont let it get you down. When you are done, you will know your machine inside and out, and it will gain your confidence.

 One other thing is if the pto circuit has an accumulator on it, you should check the nitrogen pressure in it before any testing. I believe the pressure is 100psi +/- 10. But check with your manual before you take that as fact. If the accumulator is shot, you can buy an aftermarket, over the shelf part for very little money. It is a common size and function accumulator. I bought mine from Spencer fluid power.