It could have been cut with a plasma torch. That leaves burnt edge on the hole and a fine brown coating all over the inside. (I usually fill the keg, at least partially, with water to reduce the amount of exposed surface to clean up.) To start, I would probably go ahead and open the top up to whatever dimension you want it to be and clean the edges up on the hole prior to even trying to clean the rest. Then go after it with your cleaner of choice and if that doesn't get it all, move on to the next cleaner, say a caustic like CLR. Any edges that were burnt with a plasma torch or reached excessive temperature will be prone to corrosion. That can be greatly reduced by electrolytic passivation using a DC power source, such as an old-fashioned battery charger with the negative connected to the keg and a piece of sponge or cloth in the positive lead. Soak the cloth with phosphoric acid, (sold at most home centers and paint shops as "metal prep"; turn on the charger and rub it over the heat affected surfaces until they clean up. There will be some foaming; that's normal. Electrolytic passivation reduces the iron content in the surface of the metal's lattice structure, providing a more corrosion resistant surface.
Most of the label residue can be removed with acetone easily enough.