let me tell you something.

theres a reason why the oem cat is expensive.

inside the cat are precious metals, polladium etc and they are the ones that cleans up your nox, hc,.
oem cats are dipped longer in those precious metals thereby making them more better than the regular/cheap cat that you get at a muffler shop.

i guess that oem cat is the only thing that could save you. or maybe try buying a new/used engine with low miles