The first thing to do is replace the O2 sensors. Try that. All four sensors will eventually go bad.
While under the car tap on the cats to listen for a crumbled core or take apart your pipes and look inside. If the last two cats after the second 02 sensor are bad then remove them by installing the fab speed decat pipe.
As you can see there could a total of 6 different converters on the vehicle.
There are two cats on the front cat pipe. You can cut out and straight pipe to remove the cat on that pipe that is AFTER the second o2 sensor. You could also just rip the guts out of that second cat.
I would also look at a salvage yard for the exhaust from a 2005 and later T1. They don't have a resonator for part #11 in that drawing. Just pipes.
It will be just fine with the single cat still there that has a new 02 sensor on each side. In fact it will probably run better than ever.
If the primary cat or cats are junk a replacement pipe is amost $1000 each side.
I would look into cutting out the primary cat and welding in a high quality aftermarket one.