A nice "stage 1" build to the powertrain will add some power, keep it reliable, and won't break the bank. Intake, headers, exhaust, stage 1 cams, bigger fuel pump and injectors, and a tune. But tbh, these cars are the most fun carving mountains. I'd focus more on suspension, brakes, good tires, and adding an LSD. All that makes it much more fun than pushing 400 or 500 horsepower going in a straight line.