Magnesium is not very easy to weld or "dress-up" so that it looks pretty again, as the original sand-cast surface was and also not leaving any traces of the repairs.

Below are some excellent and some not so well executed repair samples. The results achieved by the very talented fabricator/welder Mark Spraker (Euro-Tek) can be seen here; the before https://pbase.com/9146gt/image/29175060 and after https://pbase.com/9146gt/image/29296338