Sounds more complicated than I thought.
But even so, I think it could be done.
Tear the cylinder down and when it comes to needing parts, bring the torn down cylinder to your local hydralic rebuild shop. They will match up all the o-rings and seals and should not cost more than, say $40 for all these parts. You may have to leave the cylinder there for a couple of hours so they can match every thing up.
I went through the two boom lift cylinders on a 1974 Bantam excavator. Called the company that took over Bantam (forgot name) and asked for rebuild kit for cylinders. They quoted me $149 each (I needed two) and I would have to wait two months (not sure of the reason for the wait - I thought it was to give them a chance to "make" the seals). Well, I brought the old o-rings, wear pads, seals and just the gland (the cylinder was too huge to haul down to the shop) down to the local hydraulic shop. It took them about four hours, but they matched everything up and it cost under $40 total. I did not have to purchase anything from Bantam. Got some great advice from the techs there and went home and assembled cylinders. That was three years ago and excavator is still being used today with no leaks from those cylinders. Someone told me that the hydraulic shop would charge $700 each to go through those huge cylinders.
Anyway, I think you can do it. Anxiously awaiting photos. From the looks of the cylinder, what kind of wrench is going to be necessary to get the gland off?