Sounds like the clutch disc may be stuck to the flywheel from setting. If it's in a place where you can start the tractor in gear, If I remember right from one I mowed with at work, you have 3 forward gears and a high & low. Put it in what I would call 4th gear and drive it while pushing the clutch pedal down and riding the brakes at the same time. Maybe stab the brakes to shock the drive train if it doesn't want to release, just riding the brakes.
Some may suggest after getting it broke loose to block the clutch pedal down to keep it from sticking again if it sets for long periods of time. Personally, I think just using it often is the answer. I've owed 20+ tractors for 40 years and have never had one stuck and some set for a few years as they are collectables and don't get ran all that often. So not sure why some do, and some don't. Maybe the amount of humidity in the air has something to do with it causing the flywheel surface to rust causing them to stick.
Don't know if they were all that way, but the one's we had, had a 2 stage clutch where the PTO would turn before ground movement so make sure pedal is depressed enough to release the second stage. The one's we had were equipped with a 5' side mount sickle bar mower for mowing roadsides although ours were gassers. Sweet little tractors, the only problem I can think of was you had to be pretty gentle engaging the PTO using the short lever on the right I believe, too hard on the lever and it would shear a roll pin on the inside, and it would need to be split to fix it. Never happened to me but did to others and one's that didn't like to mow would shear it to get out of mowing. Just a little FYI for you if you didn't know.
