1999 B2500-Getting ready for catalytic converter replacement

[1999Mazda Central Florida]

I'm getting set to replace my catalytic converter on the '99 2.5L. It sounds like it either has a hole or it's cracked, and I have slight loss of power, but no check engine light. Looks straight-forward (2 bolts at the exhaust manifold) and a clamp with springs on other end. That whole section can be replaced with a new Rock Auto direct replacement cat. converter for about $250.
The question is: would it be a good idea to just replace the O2 sensor (since I don't think it has ever been replaced.) Are o2 sensors from Rock Auto direct replacements?
Also, would messing with O2 sensor effect the computer? I'll probably just disconnect the battery before doing anything (like all manuals say to do.)
Also, these hangers look hard to get off...just cut them and replace them with new ones?
UPDATE: Oct. 17, 2020 This is what I'm looking at getting, have to go by visual comparison...no way to tell if it's exact fit.
UPDATE: Jan. 2021 The Rock-auto IS a direct replacement and I did get 2 new exhaust manifold to flange studs. I also got new oxygen sensors and decided at the last minute to have the job done by a mechanic
considering Sgt. Sandman's warning about Murphey's law (I probably wouldn't have the tools to drill out a broken stud at the manifold,) which is exactly what happened at the auto shop
The problem was, HE ALSO had issues drilling out the old stud and wound up having to take the exhaust manifold off to drill it out and then he had issues with a manifold bolt. Murphey's law
was in effect at his shop and not my drive-way, but it still cost me $740 in labor. I also had my spark plug wires changed while it was there.

 

[Colin]

Get the Bosch brand sensors, otherwise sounds like you've got everything figured out

 

[sgtsandman]

And make sure you have a weekend to do the job. Make room for Murphy's Law. It WILL come to visit.

 

[Guanfy]

I don't know if this holds true with your exhaust layout but when I replaced the cat on my '94 I had to cut some off the flange off the pipe that connects to the inlet side of the cat, otherwise it wouldn't go all the way down to make a tight seal. Some companies will note this sort of thing when you buy the replacement cat, some won't. Something to look out for.