Get yourself a caliper off a FZS1000. Much better and more robust than stock one. Will fit standard FZS600 mounting bracket (don't use the FZS100 bracket), with a little bit of grinding down. There's only a tiny bit in the way, 5 mins with dremal or 15 min hand file will sort it. Expect to pay around £60.
Why did the bolt holes need re-boring? Was it seized to bracket?