I bent the tab to fit the one on my Dawes, but I think I had a little more room to play with between the bolt and the crown race. Some fiddling and harsh language was certainly involved, but it's fitting mudguards, so that's to be expected.
Pad it out with washers (or molish a custom spacer on your lathe) under the tab so the top of the tab can overhang the crown race? Non-destructive, so probably worth a try.
Cutting down the tab would probably work (perhaps in combination with a larger washer). Since it's clamped, does it matter if it's an open slot? I don't like it as a solution though, it's bound to break.
Nuclear option: Remove the fixing entirely, drill a couple of holes in the mudguard (or re-use the ones where the rivets now aren't) and pass a cable tie through it and the mounting hole.