With such a small engine, it will be difficult to experience much of a gain. I'd just make sure the car has good spark plugs and clean air filter. A K & N air filter might free up 2 horsepower or so since it's a better design than a regular air filter. A cold air intake would be ideal and that would free up more like 5-6 horsepower.
Realistically, the only way to achieve high gains on that motor would be an aftermarket exhaust system and forced induction (supercharger, turbo).
