I'm replacing the A + B rotaries on ours right now, actually. The B rotary is a nightmare, the accordion way covers are baffling and need extensive process photographs to put back together correctly, and it turns out there are two models of A-rotary, and they're not interchangeable. If I rebuild this rotary, it's only compatible with 51s and 52DCIs, but not 52Ds, for some bizarre reason.
Probably replace the ballscrews too, as long as this thing is totally disassembled and taking up an entire workbench. X and Y, anyways, the Z is awful enough that I'm going to pretend it doesn't exist. Gravity takes care of the backlash anyways.
Once this is all done, the 51 should be happy for another year, year and a half... really don't want to do this again any earlier than that...