I would say get it running right and then you might not need the new cat to pass. The o2 is one thing that would not hurt to have anyway but you will eventually want to get it running right and then have the money invested in the new cat that you might not have needed to spend.

I do think a new cat could get you a passing grade but again if its not running right it will probably be taking life out of that new cat. I also know how bad you want to get it up and legal to drive around though too. I just went through that with my 88, too bad I have to sell it.