While it was sitting/stored was the clutch locked in the disengaged position or was it engaged all the while. It sounds like it was locked in the disengaged position, as it should have been, and is not returning to engage position, especially if you can shift into gear without pushing in the clutch. I would try working the clutch up and down as much as possible, possibly even sliding your foot off of the clutch pedal when it is down, to try popping it lose before splitting the tractor. You should do that with the tractor not running, you will be able to feel the difference in the pedal pressure if it comes lose and the throw-out bearing starts moving again as it should. If it does start moving it may take some time and repetitions for it to free up enough to move smooth again.
The throw-out bearing slides along a collar as it disengages and it may rusted to the collar in the disengaged position.
Have you looked for an inspection plate under the clutch housing? I know they are common on older tractors but not sure about yours. If it has one you may be able to use a long screwdriver to work it free through there.
If you cannot get it lose using this method you will probably need to split the tractor.
Good luck with it