I am assumming you have replaced this bad catalytic converter with a new one, for legal reasons...
since the old cat is off the vehicle, you can cut the cat open with a saw zaw or a cutting torch, and scoop out the guts.
if you are going to put this pipe back on an "OFF ROAD" ONLY VEHICLE, you can mig or tig weld the cat back together.
I did this on a 1988 grand marquise with clogged converters that we ran in a demolition derby.
For legal reasons I would have to advise not to do this. If you are willing to break the law and gut the cat. out, all you need to do is take a long metal rod, socket extensoion or whatever. And hit the crap out of the inside of the cat. This will break up the inside and then you will just start the vehicle and it the crud will blow out. If you have an o2 sensor after the cat you will need an o2 simulator to fool the computer in thinking the cat. is good. Many people try to put in an off road pipe (an exhaust pipe to replace the cat.) this is not always a good idea as you will fail a visual emissions inspections. It has actually been argued whether or not a cat.is required to pass an emission test (I have heard of many people being able to tune the engine so well (even carbureted) that they can pass an emission test with no cat, but fail by visual inspection.) I have included some information you can read from some different forums and such. Any questions let me know and I will help you out.