It should be a pretty easy thing to change the CAT for a new one, also how old in the O2 sensor, they are a wear item, like spark plugs. Also, I think there is a small lead plug at the base of the carb covering an adjuster screw to lean out the air/fuel mixture a bit. Alcohol in the gas tank helps NOX, but might help out on CO a little as well. (I've used the HEAT product found at many chain auto part stores)

-Jon