You could file down what's left of the tab on the cover flush and mount a piece of aluminium angle across the top two screws. A bit of 2 or 3mm angle should be easy to cut down and look reasonable - just leaving a wide enough tab for the carb bolt. Longer screws in the top cover holes and away you go.
A bit of 5*5 say square bar would also do it.
I think there are some options there with some hand tools and a little time.
Is it this bit that's broken off?
