Cats on TTI exhaust system
Carbureted cars produce relatively small amounts of nox emissions, which seems like debunks your whole point.
I understand you concern I guess, but it sounds a bit like taping sponges to your feet to avoid killing the ants.
On some level the shellfish are fucked either way and you are not the problem. I had great oysters tonight. Anyways.
Just enjoy your car. Putting cats on it would likely require you to run efi to avoid burning them up, and would cost a ton.