1992 Chevy Berretta failed emissions CO reading

I know very little about vehicles but a few of my friends are somewhat knowledgeable about most newer cars but not really familiar with the Beretta.

It is 1992 Chevy Beretta and failed CO at 12.20 with 12.00 grams/mile required to pass.

HC tested 0.63/1.00 and NOX tested 2.43/2.50 both of these passed but figured I'd put the information in here along with the CO in case if it can help pinpoint what may need repaired.

What has already been done to the vehicle since December 2011:
New Spark Plugs/wires
New Catalytic converter
New O2 Sensor
Vacuum leak was fixed
New Air Filter

What else could affect the CO emissions that may need to be repaired/replaced? (Looking for something that won't cost over $450 IF possible.)

Some things to mention:
1.) Head gasket has minor leakage.
2.) Car does idle rough when stopped while in drive but is just fine in park. Smooths out once it gets going again.
3.) I was sitting in line at testing center for 30 minutes, I did rev engine however the car may have cooled down/catalytic converter may not have been working at it's best.

I just want it to pass emissions so I can the vehicle titled in this state. Thanks.