1990 Honda Civic failed smog check for high HC, CO and NOx.

If this Honda Civic is the 1.6 liter with a non-EGR equipped engine than you might want to consider the CAT, BUT ONLY after an accurate vacuum and compression test (since the rings are new) and a you've verified the computer is in proper fuel control. You want to do the engine mechanical tests first (visual inspection; making sure everything is plugged in, compression and vacuum tests... so on), then you should check all the main emission sensors, such as the TPS, ECT, MAP and O2 sensor. And then if everything checks out, do a CAT test. There are a few different way to test the catalytic converter. They all require a  5 Gas Analyzer. A home test you can do (if you own infrared thermometer) is checking the temperature of the exhaust pipe right before and after the CAT. You should see at least a 40% increase in temperature directly after the CAT as opposed to the front of the Catalytic Converter.