Ages ago, you might have had some cylinders that had V-Packings, and you could screw tight on the washer to kind of make them fill up the space again—but they had high leakage, not to mention high maintenance, and they’re just a pain.
If you can source those, the best thing to do is have a good relationship with your hydraulic distributor and they will likely have a good source of seals themselves. You might be able to get some of these old seals. Chances are, you might have to upgrade so either find the cross-section that most closely matches what you’re using and see if it fits in the pocket that you have. Otherwise, if your cylinder is so old, if your pump is so old that you cannot get seals for it, perhaps it’s just time to upgrade. Get a new cylinder. Get a new pump.