have a read of this
Mini Dirt Bikes & Pit Bikes Forum : MiniRiders.com.au - Porting! pictures do not work but it will help you.
I have just ported a head and I cannot belive the difference it has made, myself and a friend have the same bikes and in his words "its like going from a 250f to a 450f".
I can post some pictures later as its out of my bike at the moment.