Let's start at the beginning. Do you currently have the TRD header and the HKS muffler? If not, TRD headers have been discontinued for some time, so you may not find one easily, and HKS makes the muffler in supercharged configuration, so you might get that instead.
If you already have the parts, I can make you a cat that will fit your header and muffler perfectly. If I get all the info I need, it will be a direct bolt in. If I use a standard Magnaflow blank it will cost you about $125 for the whole thing (figure $135 shipped to a US address). I will want to see the flanges on the parts you have to make sure I have all the information I need. Can you post clear, sharp pics of the flanges, and measure the inside diameter of both the header outlet and the muffler inlet pipes?
Ideally, you'd want to carry the larger tubing diameter all the way to (and through) the muffler. You can do that by selling the NA muffler and buying an SC version. If that is not practical or economical, you can have your local exhaust shop cut off the muffler inlet flange and expand the pipe to 2.25" so that you have a smooth transition to the smaller tubing. I can also include an extra flange that you can have a local shop weld on if you go that route. They are $4-8 each depending on which style you need.
Here's a cat and downpipe I made up for another AW11 owner
There are more pics of the downpipes I've made here if you want to take a look:
20v exhaust