Couple of fast road cams, sports exhaust (Scorpion is my Rover Fave) and an induction kit (with a cold air feed) will do quite a bit for you. Those engines are pretty solid although can be a little Torque Shy low down - I guess thats the Honda heritage though. See what Moto-build have to offer in the way of cams.
For budget tuning get a panel air filter and cut 4 inch wide holes in the bottom of the air filter housing (use a spare from a breakers yard). If you are planning to change the engine then I would concentrate on suspension and handling mods for now.