Over boring is not going to add horsepower on a 2-stroke. If you wanna go faster, look into porting and adding an aftermarket cool head (pro-design, chariot performance, NOSS) to name a few.
Obviously, a 4mm, 7mm, 10mm, ect. aftermarket crank is going to add horsepower also but just remember this. All of these parts need to work together. Don't just go throwing shit at it and "think" it will work. If you aren't mechanically inclined and have a few extra dollars to spare, call a site sponsor and discuss what you are doing with the bike and they will put together a motor packege that will suite your needs.