I've put two Peco exhaust systems on my own MGs, and helped put a third on another MG. I'm not sure how the Abarth exhaust stacks up, or if you can even get an entire system, but I can attest to the quality of the Peco. (Note, I run the Peco header, too).
Make sure whatever oil you are running is suitable for an engine without roller tappets, else you'll end up wiping your tappets or your cam, or both.