If the max rpm is at 9000 then max hp rpm is going to be around 7800 to 8200. So you should figure your max speed at 8000 rpm where max hp is. Realistically you probably aren't going be able to go beyond 35 mph with a 2 stroke 42cc engine.
At 8000 rpm on 27.5" tires you should use around an 18:1 reduction. 10:40 and 11:50.