"is it possible to to just cut off the catalytic converter and replace it with a new straight pipe and muffler?"
Anything is possible.
Legally, since the cat is part of the emissions system, it is supposed to remain intact.
What you can do, USING EXTREME CAUTION, is unbolt the catatlytic converter flange at the front of it, pry the flange open and wedge a piece of steel in the flange to hold it open.
Start the vehicle and take a short road test. Use caution, as this creates a fire hazard.
If the vehicle runs out ok, then you know the cat is plugged.
Again, be aware that this creates a very real fire hazard.
Have a fire extinguisher on hand and make sure the underside of the vehicle is clean of debris.