Hucker.
If your case is magnesium it should say so somewhere on the case. Tig welding would be your best bet. If it is mag, then use mag rod. Magnesium is hard to weld because you have to "Boil" all of the impuritys out of the metal.
Like BigLou mentioned, you can grind the crack back to clean metal and clamp it to a flat table to prevent warping. If your not sure if it is aluminum or magnesium, clean the weld area using a stainless steel brush until it is bright and shiney. Weld the crack using aluminum rod. If it is mag, the weld will not stick after it cools down. Chances are, the whole weld area will just pop off of the case. If that happens, clean and grind it again and use mag rod. Cover the case and let it cool down slowly to prevent cracking.
Another way to tell if it is mag, try grinding an area of the case with a rotary grinder or belt sander. If it is aluminum, you will have small chip like grindings. If it is magnesium, your grindings will be more of a fine powder.
Just my $ .02