I use Tremclad High Heat Enamel in spray can. It is rated to 1200F. The flat black dissipates the heat into the engine compartment, while the flat white holds the heat in, allowing the O2 sensor and catalytic to get up to operating temp more quickly.
90% of the time a P0420 is a bad cat converter. If you want to throw in a new upstream O2 sensor, fine. But you'll probably be replacing the cat converter anyway. See this post on P0420