In order to rebuild the cylinder, you would have to cut the end off of it, source new seals from a hydraulic shop, then weld the end back on it. The other PITA is that the tie-rod ends are welded onto the cylinder. If those are bad, you'd have to source an appropriate TRE, cut them down to fit, cut the old ones off and weld the new ones on.
It's either that or buy a new one from a dealer.... I have not heard of an aftermarket source for these. It woul be nice if there were an aftermarket source.