I bought my 2005 R53 has a decated Madness Motoworks v2 header. I want to install a cat in the system for emissions and noise level reasons. I couldn't find the dimensions of the original catalytic converter to find a suitable replacement :(

I am currently looking at the Magnaflow universal cat 51206, which has a 2.5" pipe diameter and is slightly shorter than the OEM cat. Is it going to fit? If not, what are your recommendations? I'm trying to avoid changing the whole header if possible.

 

I can't speak to the Magnaflow, but if I were you, I'd call a couple local exhaust shops and see what they quote you. A good shop can probably weld you up a cat and some flanges to make it a bolt-in affair with your header for a reasonable price.