I tried to cure my oil leak with gasket goo.. didn't quite work. Still oil in the well and issues after rain if the car had been left for a few days. The last oil service cost me a fortune.. oil/filter/rocker cover gaskets/coolant/belts.
From hindsight.. make sure you replace things that need replaced!
If i wasn't so busy and had workspace.. woulda done the job myself.. but its not that difficult. Just a bit tight at the back of the motor for space to get the cover off completely.
If you are misfiring.. then yeah the CO levels will be higher.. but the DME will compensate and lean it out to get better results.
Does it have cats?
Yes... i'm a total car nut! Why else would I upgrade my mower to rev to twice its standard rpm!