You can "gut" the cat basically by taking it off and banging it on the ground until the assorted stuff inside falls out. Voila, now you have a straight pipe. Of course, this is not emissions legal but it'll probably get the car running smoothly again. I would recommend you get high flow cat. A Carsound high flow cat can be had for probably 100 bucks or less and then you just need a muffler shop to weld on the proper flanges to bolt it in place of your stock one. Otherwise, Landspeed Racing sells direct-fit high flow cats for $230.