A few things gleaned from the antique tractor pages...
Try to keep the clutch pressed in for a while, ie days. Press the clutch in, and then wedge it with a 2x4, or baling wire, or something to keep the pedal down.
While the pedal is pressed, try rocking the tractor back and forth while in gear. You can get a lot of leverage when rocking the rear tires.
You could also run the tractor, in a low gear, while pressing in the clutch, and pressing/releasing the brakes.
Of course, this is for a gear transmission. This could be a predicament with a Hydro...
RobertN in Shingle Springs Calif