Just buy the GYT-R from Yamaha, you can find them on EBay. Switching to a 13/39 will be like a rocket off the line and accelerating out of corners, climbing, jumping etc. You will have to shift quite often.
When you do your upgrades look for the lightest components possible.
Every 7 pounds of weight you lose is like adding 1 HP.
For your drive line it only takes 10 oz to gain a pony.
We built our quad with this in mind, but in the end it is never enough and motor mods are the only answer.