Considering it is a Type R engine, the cylinder head already flows pretty well. But, there's always room for improvement. If you are trying to get as much as possible out of the engine while staying all motor, its going to get expensive. You have a lot of options:
Intake manifold
Exhaust
Intake
Cams, valvetrain
Larger diameter valves
High compression pistons (though your engine already has a pretty high CR)
Head work (porting, polishing, etc)
Most importantly - a tune to go with all or virtually any combination of these parts
Like I said before, simply throwing parts at it will not yield the best results. Research and a well planned build will net the best results, and end up being the least expensive route in the end.