I don't know the answer to the question but remember that by shortening the distance from the crank to the cam by milling you are changing the cam timing. IMO this would be more of a concern than the piston.
i dont think you would be changing the timing and if it did it would be so small. The one thing you might have to look at is are you still going to have the correct valve to piston clearence, so they dont bang. There is a guy here were i live that can fix/repair just about any bike head out there. JSR Performance in Wichita, KS. Right now he is trying to get there Pikes Peak racer ready.