Simple Green Pro works well and doesn't hurt aluminum. If possible, I'd put the case in a large tub and boil the thing on your BBQ for an hour or so. As a pre welding step, acetone, carb cleaner, brake cleaner, something nasty and harsh to get any remnant oil out. There will still be oil in the porosity of the aluminum, and your welder may cuss you for it. But at least you will have done an honest job of minimizing his agony.