You could replace with an aftermarket spinner, as long as you attached the existing Atari optic boards to the new spinner. (or other compatible optics, like the ones in B/S/T) But (IMO) it would be a shame to lose the original spinners an their Atari-logo'd knobs on a nice dedicated machine. Sadly, Blasteroids knobs fit a different size shaft than pretty much any other spinner out there, so you can't just swap the knobs onto a new spinner, either.
I would advise dis-assembly, cleaning, and lubrication to see if that gets performance up to par, first. But if you do decide to replace them, you probably won't have any trouble selling the old ones in B/S/T here.