3 options mate, depending on how the CAT is fitted at the moment.
If its a bolt in bolt out pipe then its easy - just replace the Cat
If its built in to the front pipe, then you either need to:
Get it cut out, have flanges welded in and a straight through pipe put in.
Have a new front pipe made up without a CAT in it.
Peco down by the dock road in Wallasey / Birkenhead should be able to do any of the above for you in stainless