You’ve never touched the sparkplugs, wires, distributor cap, rotor, and (I assume) filters for years? No wonder you’re failing! No wonder your gas mileage is dropping!
Before you try to “fix” the car, get all of this stuff up to date. Then see what your emissions readings are.
Be advised that your high NOx readings may be a result of your lack of routine maintenance. Excessive NOx is formed by high coombustion temps, and the two primary methods used to control it are the Exhaust Gas Recirculation (EGR) system, which displaces a bit of incoming oxygen with inret exhaust gases when the engine is under load to keep temps down, and the catalytic converter, which contains a catalyst that when heated seperates the nitrogen and oxygen. Both of these systems can be “poisoned” by carbon deposits, which are common byproducts of incomplete combustion…which is a common result of poor maintenance. The catalyst can be coated and made ineffective, and the EGR system can get gunked with carbon and made ineffective.
In short, get all the maintenance u to date. Then, if the readings still fail (and they probably will) let the tech check for trouble codes and determine why. It could be that you’ll end up getting a new cat converter, but don’t guess. Let the diagnosis be done.
Oh, and getting everything up to date should help your gas mileage too.