Old cats don't die. They are poisoned. Something has coated the catalyst or has killed it. You should be wondering what did it in. My old truck could buy beer, and it has the original. My old car has enough mile to go to the moon, with the original cat still catting.
I don't know 420, but if it is indicating poor cat performance, it is done using a second O2 sensor after the catalytic. Sometimes it is the O2 sensor that is failed, instead of the catalytic, indicating poor performance where there is actually no problem.
Why did it die, if indeed it is dead? NAPA should be able to provide a bolt-in replacement, or you can possibly get a generic from JCWhitney for about half that (well, you could a while ago)
tom