You won't really know how bad the damage is until you clean them up.
I'd try a bit WD then a rag to start with, failing that, your best chance is likely to be to remove the nut, and use some electrolysis, as it'll dissolve the rust without resorting to abrasives.

All you need is a suitable container (gutter section works well for long narrow things), and a suitable power supply. It's not something I've personally done, but I've seen some remarkable results on a couple build threads in the past.