IMO for better power, and maybe help mileage, open up the exhaust and intake path. Meaning headers, duals or better flowing y pipe or main pipes, like bigger dia, less restriction is what you are after, then like a cold air intake or something to open up the intake side. salad bowl mod or something to help get the air in better.
if it aint done already, a good tune up, plugs, wires, rotor and dist. cap. maybe even pop on some e3 plugs, the guys on horsepower swear by them to help make more power and less emissions.