Porting is a good mod to perform on a motor. But be careful if you don't have it done right you would be waisting your money. Porting gives a smooth flow to the valves, and increase horsepower mostly in top end. Most of a motors horsepower is creaed in the head. A good port job, valve combination and a good cam grind mwill make ur puppy run. I would go to T.c , Lrd, or any of the top engine builders. Mostly all of them have experience with the yz head and should do a good job satisfying you!
It depends what your looking for.You could reaplace the cams. you could big bore it, stroke it , or just up the compression too. I t just all depends on how fast you want to go . BUt reliability will start going down if you do too much .