Yeah you could weld that up, but be careful about how you run it from now on.
Whatever caused that crack has also weakened the aluminum surrounding it. talk to your welder about lightly reinforcing the metal surrounding the crack.
You don't want to over-strengthen one part of the case. If you do, then it will not absorb stress like the other parts will. So the stress that would normally be distributed evenly throughout will be passed on to the weaker areas, which will cause a stress-concentration and break more stuff.
Make sure your belt tension and boost levels are on the lighter end of things. You are already on thin ice!