Intake and exaust parts, you can also use the cam and head parts to make it a 2.7i. You may be able to get these off of parts cars used very cheaply. A better differential helps too, a 3.25 LSD would make a difference in acceleration but would raise your revs a bit. Also what Mel said about chip and filter, and wheels. No matter what you do to your engine, gas mileage would go down. Also there are ground effects, air dams, and skirts that really sport up the look of a car.