It is easy to remove the cat in the headers, remove end can and if you shine a torch down the hole you can see it, looks a bit like
a honeycomb. Just drill a lot of holes round the edge of it and then hit it a few times with a drift and most of it will come out. Then dremel the rough edges off and before you put end can back on start engine up to blow the crap out. I used a magnetic probe to get the bigger bits out first.
There are a couple of vids on youtube that show how it is done.