If you have engine oil on the area, you can also use covering heat gun to heat the area, the oil will come to the surface, and you can blot it off. Once you can't blot any more off, then use the cornstarch and let it set on the spot for a while, and when you remove it, it would have some more oil in it. The more oil you can remove before applying the balsarite the better off your replacement covering will stick. You may want to try and go over the area 3 or 4 times with the balsarite as it will also help to seal the surface from absorbing more oil in the future. I do this to my glow planes all around the engine compartment and under where the fuel tank goes. I have had a couple of times the bung on the fuel tank develop a leak, and there is a good quantity of fuel that can be absorbed by the balsa and ply structures in that area.
Good luck!