If it sat, the clutch which has friction material that can bond to the flywheel from lack of use, he's probably saying that its been stuck before, best thing to do is block the clutch pedal down when not in use. If stuck, you can usually get them free, but it requires starting the tractor in one of the higher gears, (maybe not the best thing to try for an un-experienced person, given the safety aspects) Once running, working the brakes, putting a good draft load on the drive train, say with an implement like a subsoiler, moldboard plow, if it had a loader- push up against something substantial, work the brakes to try and slip the clutch, break the rust bond. Once a clutch seizes up like that, its prone to do it again, a new one installed, flywheel surfaced, solves that, but you can't let it sit, it needs to be used, and if it has a 4 speed, the rubber boot on the shift lever, if faulty can leak water into the trans compartment on these. Easy fix, but its best to keep a tractor under cover.
Clutch installation requires a split of the tractor, with no loader, an experienced person, properly equipped should be able to do it in hours. There's a few things to know about both splitting it, aligning the new clutch and reassembling it. I could be considered a signigicant amount of work to get a new one installed, but should easily be done in a day by an experienced mechanic.
Over running clutch is a part that is installed on the PTO shaft on the back of this tractor, some call it an overunning coupler. It allows for the implement on the back, the mass/inertial of a something spinning to not power the a pto directly powered by the transmission, depending on your model, if yours has as single stage clutch, you would want one installed for say using a rotary mower. What happens is you depress the clutch, while the transmission is then disconnected from the engine, the implement on the back will power the transmission from inertia, the overunning coupler prevents this. The safety issue is that while the operator of the tractor has depressed the clutch to stop the tractor, it will keep moving until the implement stops, so you could run into something, go into a ditch or similar as you are expecting the tractor will stop, it won't until that implement winds down. With a rotary mower on, all that mass keeps it turning for a short time, long enough to be a problem. They can be bought for less than $100, maybe $60 or so, well worth it, and easy to install.
The live pto, its possible, you'll need a bunch of parts and thats not going to be easy. You would be better off changing the transmission out to one that is set up for the 2 stage clutch, but depending on what you find, there are some changes in shaft splines on these, you will have to be cognizant of that when sourcing parts, know count spline is in the one you have and match it. In a sense, it might just be easier to swap in the entire back end, trans, hydraulic compartent and differential. Realize by changing the transmission, the original stamped serial number of that tractor goes with it, its on the flat area above the starter. A fair amount of these are mutts by now, but you may not want to do that with a nice original.
If you have not run tractors before, you had best learn about safely working in your conditions. If you have hills, they can be dangerous, you do not ever want to turn one over if it does not have a roll over protection structure and a seat with a belt. If towing logs or similar things, that hazard will be ever present and there have been topics here about fatalities of new inexperienced owners, don't be one of them, be safe and do not take risks for any reason.