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Welcome to AF/Purehonda.
Predator already said most. But wathever you do, it will be expensive. Their isn't such a thing that's very cheap and gives you tons off extra hp's. You can go with a complete exhaust setup (header, cat, exhaust), but this can easily go up to 1000 euro and it will give you 10-15hp. A CAI is an option, but you should look out with this when it rains. I already heard some people who had it and drove trough water spots and killed their engine.
Since you have a DOHC, it might be an option to change your cam gears. I don't exactly know the price of this, but if they are placed and tuned on a dyno, they will give you easily an extra 10hp.
You can also put in other cams. But if you do this, see you get enough info on them. The ones for race driving are really bad on the road (you will have no power in the lower rpm range), you also have semi-racing wich are a little better and you also have the sport ones (which are best for street use I think)
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