There are many ways to add horsepower- it all depends on how much money you want to spend
Heres a list of things that would improve performance:
with the biggest increases in horsepower at the top:
Turbo Charger +20%HorsePowew(BHP) (Yes ppl put them on bikes ) $1400+
Power Commander + Exhaust Pipes +End Cans 5-15BHP $1000+
Exhaust Pipes + End Cans 4-10BHP $500+
Port & Polish 2-3BHP $300+
End Cans 0-4BHP $300+