You can buy abs rings separately. I've changed several, including on my own car, a volvo.
Probably depends on the make, but on some cars yes you can. I've replaced 3 rings on my Ford, they only cost a couple of £s but are a pain to fit.
As for the ABS failing an MOT it'll also probably invalidate your insurance so worth sorting. I understand the annoyance but ABS in general will make the car safer.