Be sure to take out the drain plug when you weld it, but stuff in a bit of cloth to fill the hole a bit.
The oil inside may go on fire with the heat of your excellent penetration welding, and if the plug is in it may blow up a bit and spray you with flames. The standard vent won't be big enough to save it.
Take care and be prepared for a sheet of flame spurting out. It will self-extinguish inside.
CharlesY