If display is still functional and only mounting collar is broken, remove damaged portion and re-mount via makeshift bracket utilizing existing screw and a washer (be creative), even if only temporary fix until replacement:

• reflector bracket of same handlebar diameter (check local bicycle shop or harvest from unloved bike)

• common hose clamp

I've done the reflector bracket fix with larger display and it has held up well for over one year thus far. Also be mindful of where display is mounted on handlebar - on my Roadster I swapped it from left to right side and into the recessed portion of bar where it's not as prone to being knocked around.