Well the new fancy air filters on the market are suppose to increase horse power some what but funny at this moment I can't think of their name lol
Using a oil treatment believe it or like Lucas can greatly increase some engines horse power or even a gas treatment or additive such as Lucas can over time increase horse power.
You might find this all funny but even a fresh gas filter or oil filter or oil or transmission filter or transmission fluid or transmission additive can increase over engine performance or changing the plugs and wires can lead to increased horse power too. Example one of the best plugs around has a more steady spark and thus increases not just mileage slightly but horse power slightly too.
Hope that helps and best of luck.By the way really these are basic simple things that aren't very expensive and all proved to increase horse power in engines in general.There are other tips that a good car jobbers can help you with but that's the basics that I've mentioned.