Weld away, I would just use some silicone, but the crack might continue
to spread and that would result in a total failure of the case or just more
leaking, you will have to drain the case over night to get all the oil to drain
out, it would help to have the gear oil warm to drain out better, then clean
the weld location good with brake cleaner so it can be welded without problems
Good Luck
Wild