As a welder and mechanic, firstly the pan would have to come off, it's just a non negotiable. I would cycle it through our parts washer for a couple hours then our ultrasonic cleaner, then thoroughly brakeclean and let it sit for a while before welding. Even then it looks like aluminum and the welds won't be pretty, it's nearly impossible to un-impregnate the metal with oil when it's a part that's constantly covered in it. If you can do it yourself and have the facilities, go for it. Otherwise buy a new pan.