All bikes have clips and rubber block spacers holding the guard to the frame cross tubes. Earlier versions have clips and an angle bracket (42 6820) to the frame as a lower fixing. All depends on what combination of mudguard and frame you have. With a bit of luck the holes will line up and you can choose the most appropriate fixing.
The rear stays provide the basic datum, spacers to the frame need to be made/arranged to fix the guard relatively stress free otherwise vibration will result in fatigue cracks and splits.
Swarfy.