buy a frozen boost free flow cat, its like 40 bucks.
no way id reuse a cat thats smaller then the down pipe. or a used one either.
and you can always have the new cat welded to the dp out of the car at a welding shop. i got a few local iron railing and fab shops i go to for stuff like this. if you bring them a bung they can weld that in too.
i think they look at it like 20 bucks is beer money lol it takes no time really.