Well first off (trying to save you a little £££ first) empty the oil out, when that's done you need to know just how big or small this hole is, if its rust (doubtful) or a stone or rock may have hit is and made the hole, stick a screwdriver in it and poke it around looking for any weakness in and around the hole, if you do find more damage then at least you know how much to take off make neat and then weld around, if its just the small bit then, why not weld around it, best to take the pan out and clean all the oil out first, you don't want to start a fire do you, i see no harm in doing that, as long as you do the job well and its waterproof, then in my opinion carry on m8