The crankcases are so thin becuase of weight probably, they arent supposed to be durable against rocks and the ground; thats what skid plates are for. If too much heat is put on the cases by a torch, then they will warp in that spot, causing the two halves of the cases to not properly seal in that area, causing you to leak oil.
How did the the drain nut get ripped off, from casing a jump??
If there is a hole in your case that needs to be welded, then you'll probably be better off buying a new half.