Um.... I dont know if you would plan on porting the sparkplug hole... or maybe the stud holes. Hmmm :-?
My point is its a 2 stroke. How fresh is the motor?? Sometimes just doing a top end will bump your compression back up enough to make it feel like it has twice the power. Otherwise... save the money and put it towards the next bike.
My son just moved up to a YZ125 this year and we had YZ85 before that. Put a V force on it port the cylinder mill 5 or 10 thousands off the head a pipe silencer and jet it right. Other than that maybe revalve the suspension and tell him to hold it wide open. Because the Yamaha YZ85 run best on top end not a bike with very good low end. Just my two cents!!!