K&N filter with a cool air feed and preferably a heat shield,modified throttle body and manifold(have a look on E-bay germany for tasty lexmaul ram-air manifolds),4 branch exhaust manifold,fast road camshaft(with avernier pulley and duplex valve springs),ported big valve head etc etc.Its a question of how much you want to spend,even a good engine service(oil&filter,new air filter,plugs and leads etc) can make a noticeable difference to a neglected engine.One good point to note is make sure your engine is in good condition before you start to tune it,as if its a bit tired it wont respond to tuning as well as an engine in good nick and could fail do to additional stress imposed on it.
