I have lost count on how many cases i have welded up with holes and some the size of my fist.
It is no good using any type of epoxy fillers as, they always fail in the end. To do this repair correctly the case needs to be bolted down to a inch and a half block of steel to stop any heat distortion from the alloy welding and then welded up a little at a time until it is fully built up, then it is a lot of grinding, milling the matting surface and clean then paint.
You will need to have enough clearance from the final drive cog as, this sits very close to the rear of the cases.