1997 Suzuki Sidekick Check engine codes P0340 and P0420

Year: 1997
Make: Suzuki
Model: Sidekick Sport (USA)
Engine: 1.8 L MFI V4
Transmission: Automatic (4x4)
Mileage: 145,000

Issue: Check engine codes P0340 and P0420. Inspection expires in April! (Please help!)

Code P0420 - "Catalyst efficiency below threshold":
- Check engine light has been on a while with this but I haven't worried about it too much since it's just emissions.
- I've been told it's a catalytic converter that might need replacing. Any suggestions? I've heard replacing the converter can be really pricey. Could it be anything else like an O2 sensor?

Code P0340 - "Camshaft position sensor circuit malfunction":
- This is a new code that I've known about for < 1 month.
- I'm shocked that this code showed up because I had a mechanic replace my camshaft position sensor recently (January). My car was dying on me, then wouldn't start for 30 minutes. I haven't had issues with the car dying since he replaced the sensor, but now the there's a code for the sensor?

I'm open to a lot of different options. Any help with either problem would be really appreciated. I'm hoping to give the car to my little sister in a few years when she starts driving. And I want it to still be running! :)

Thanks!
Robert W. Berry