I assume the pedal can't be pushed down? If that's what I'm understanding, you have two options. #1 (the easiest), follow the linkage to where the arm goes into the transmission case. Should be a rod that pivots in the case itself. Spray it down with penetrating oil and keep trying to make it move. Might as well spray all of the linkages down. #2 (the hard and "correct" way)....split tractor, remove the clutch release fork from the actuating rod, remove the rod from the trans case, clean the corrosion/rust off, grease it up and reinstall it.