As my father use to say "if it ain't broke don't fix it". For 98 model take it down to autozone and have them run an OBD-II scan on it for free. If the O2 sensors are going bad, or are bad they will throw a code that the scanner will pick up.
If no error codes come up, then start with the usual suspects, frequently overlooked items like a new air filter, oil and oil filter change, new plugs, wires, and especially a new rotor and cap on the ignition system. Then have a muffler shop check the exhaust system for flow resctrictions. When Catalytic converters and mufflers start to go bad they start putting excessive back pressure on the engine exhaust causing poor mileage.
