Providing you aren't losing oil or taking water into the box I would guess you'd be pretty safe to use it, but I would get it looked at by your local service centre and from my experience you'll probably get some, very helpful, free advice. As for a repair....be careful, because welding cast alloy is difficult to do properly and because of the close proximity of the bearing housings, the alloy may distort. But that said, there are Engineering companies around that can weld under water to stop the heat distrubution. It will mean stripping the box though!!