Toystory say:
It would help a little to know the rear tire diameter, but I agree with several things said above. With your current 5:1 ratio, you're pushing your luck already as far as higher gearing.
Read up on governor removal- there's a TON of threads and posts on this. Also- you don't have to remove the governor to buy a few extra RPM. Adjusting it can bring you up as high as 4000-4200 RPM which will give you a few extra MPH. Look at the factory hand throttle that's mounted to the engine. Behind it, you'll see a screw with a spring wrapped around it. You'll see that when you push the hand throttle to full, it rests against this screw. This screw sets the governor's upper limit. Removing this screw completely will allow maximum GOVERNED RPM to increase somewhere between 400-600 above the spec 3600.