That's similar to what I had thought Peter, I see no other way.You can try to thread the holes with M8. Start with a regular tap and then end with a broken one you did grind flush Or drill the M6 hole 8.5 mm for a couple of mm Insert a set screw in the threaded M6 hole and then push against that with a M8 bolt
BUT The bore of that ring is a acurate fit with the spindle and rest of the bore So does it run smooth after reinstalled ????
Let us know please
And, regarding reinstallation, my idea was to install the ring without bolting it down, put the quill in deep in its bore so as to align everything and then tighten the M6 screws.
One should, in principle, be able to pull that ring off in order to, e.g. replace the lock ring I guess....