my nickel's worth: think welding is always best but not wouldn't hesitate to try Marine Tex. It's been a while, but I've also had good success with West System epoxy thickened with aluminum powder -- recommend that if you have any on hand. (Expensive for a small repair -- wouldn't go out and buy it for just that, but good to have around the shop.)
Whatever your fix, I would pressure and vacuum test the gear case before refilling. Not much pressure (maybe 8 or 10 psi).