There are four promary causes of NOx emissions (NOx, BTW - is "Oxides of Nitrogen." NOx is a contributing factor in Acid Rain, and it not the same thing as Nitrous Oxide, either.)
Lean mixture - when the mix is lean, combustion temperatures rise. When the mixture is correct ("Stoich") or slightly rich, some of the heat energy in the combustion chamber is released as the fuel droplets in the chamber are evaporated. Even with fuel injection, there are droplets in the atomised fuel mix.
Thermostat too hot or not flowing enough - This will delay the beginning of cooling the chamber walls, and will also manifest as a higher base coolant temperature. Not cooling the cylinder enough will also result in higher chamber temperatures.
High Mileage - with smaller engines especially, high mileage engines build up a layer of hard soot (carbon deposits) on the face of the combustion chamber in the head, the valve heads, and the piston head. This not only raises the base effective combustion ratio (resulting in a higher initial temperature!) but also slightly insulated the only cooled part constantly exposed to coolant flow - the head. Running a cleaner to knock the Carbon deposits out can help - but disconnect your catalytic converter while you are cleaning to prevent clogging. Replace or reconnect after cleaning the chamber
Spark Plug too "hot" - Spark plug heat refers to the length of the path that heat must take to leave the spark plugs. Hotter plugs have a longer porcelain nose behind the threaded collar, and retain more heat. Going to a "colder" plug (shorter path) can help to minimise this. If you parts guy is worth a damn, he will know what to get you.
Note the common factor in all these - HEAT. An EGR valve is supposed to help with NOx, but it is not a substitute for real engine tuning. Pity they force these things on us, as EGR valves are directly responsible for increases in CO and HC emissions due to imcomplete combustion (contamination of fuel.) Many devices on an engine designed to reduce a particular emission gas result in an increse in all the others...
BTW - Find a dealer for Beck/Arnley Worldparts - they may have an EGR Valve for you...
FFZ