Buy a universal that has the same inlet and outlet pipe size as the one you are removing, and an overall length that is the same or close. Cut the old cat off as close as possible, slip the new one onto the pipes and clamp it in place with new exhaust clamps (unless one of your friends can weld it in.)
your local auto parts store can probably help you find a universal that is close in size to the direct fit, check your local NAPA, Autozone etc..