Microsoft Surface magnesium case broken, can I repair?

Well since the surface is a fairly new product i dont think there are any replacement cases for it, although i dont imagine there will be any magnesium cases except those made specially for microsoft by component manufacturers.

You could try to contact customer support to see what would be the cost of replacing the case. Im guessing your still in the warranty period so user teardown should be considered as a last ditch solution.

Also if the break is not that bothering you could try to cover it up with one of the cheap surface covers available on the net. Until cheap backcases become available.