Two part question about a 1990 Ranger 2.9

This old beater has been running great until recently.

Air volume: The factory airbox deteriorated and had to be removed. I replaced it with a K&N cone filter, its been running rich and now its taken out cat number 2. My thought was to paint part of this K&N cone with bedliner stuff (or something, open to suggestions) to block volume. Im sure I can find the volume of the cone, wondering if theres any way to know the volume of air the stock configuration lets in. From there I could guesstimate what percentage of this filter to block off.

The Cat: The cat is shot again, making all kinds of racket, feels plugged at times. I dont get tested here and dont ever want to buy another one... Whats the best way to clean this one out and should I worry about losing back pressure after its cleared?

I appreciate any insight.

That's not what is causing the issue. Check all of the sensor connections, is the check engine light on, is debris in the intake.

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2014 F150 xl 3.7 6 speed auto.

I replaced three sensors, all codes were cleared (O2, MAF, TPS). However the air intake temp sensor reads 0ohms so I'll replace that too.

The IAC was cleaned and seems to function but that's possibly bad too.