I don't know how bad yours is but I have fixed a lot of these that were in very bad condition.
I do this by pulling them completely apart, cleaning everything to perfection, polishing the piston, remove wire wheel and clean out the bleeder valve, and reassembling them.
I have even reused 25 year old seals and they worked perfectly.
Yes, I would rather have new seals and piston, but if you can't find a rebuild kit....