To get to the piston unfortunately you need to remove the heavy cross bar which the drop links attach to at the rear of the tractor and also remove the banjo bolt which supplys the oil at the front of the cylinder. There are two cast units which have 4 bolts at either end of the bar, if memory serves you just need to undo these and the banjo bolt at the front of the cylinder, take the cross bar off and the whole cylinder should slide out. The cylinder and piston are on the nearside of the tractor.
I would try running the engine at high revs and moving the control lever up and down first as this can sometimes cure a sticky valve if that is the case