Catalytic converters can go bad pretty quickly when engines are out of tune. p0420 is possibly that.
The definitive test is a pressure gauge where the O2 sensor is.
If the compression on the motor is good (compression test) you need to look elsewhere. If compression is low on all cylinders - timing belt jumped a tooth.
Vacuum gauge will also give you an idea. If, at idle, the vacuum slowly goes to zero, you are no longer 'pumping air' and also points to a clog.
I had one motor loose power when 3 rocker arms jumped off (Chrsler 4.7, overhead cam) Compression check was weird, leak down test was also weird. I suspect owner didn't change the oil often enough and hydraulic adjusters (like lifters) were all coked up/clogged. Reved it too much when cold.... No noise from engine, no check engine line, just way down on power.