How can I fit this mudguard?
Use a hacksaw or a dremel tool to zip out a notch in the tab like this:

This will allow you to slide the fender onto the brake bolt and tighten everything down without losing vertical fender adjustability. If you wanted to add a bit of security, bend the very ends (maybe 2mm) of the tab in a vise so that it can't slide out (though, if your front brake has come loose, you might have worse problems than a rattling fender.) Make sure if you bend the tabs that you bend them away from the fork crown - sometimes you have to do this anyway to get the tab to clear the lower headset cup, but you get the idea.