If you straighten the ring (e.g. in a vice) it will work fine. As David said it just needs to get a signal, and the gap must be within the tolerance.
If the front ring is defective it's also not a big deal, the ABS will simply shut down and the warning light will come on. It will under no circumstances apply the brakes. I have done hundreds of kilometers without the front ABS ring and as soon as you put it back on you the ABS works again normally.