I have a 9.9 4 stroke on a 2011 Avenger and it worked great on the 2006 Avenger also. With the 6 wheel vehicle i would lean more to a 5hp 2 stroke. They are light and will push very well in the water. Even with a 9.9 there is only so much speed that you can get due to wheels, hull design and so on and so on. You will need to get a long shaft motor, the short shaft will not reach down far enough to prevent cavitation.
Lewis
Life should not be a journey to the grave with the intention of arriving safely in an attractive and well-preserved body, but rather to skid in sideways , cigar in one hand, whiskey in the other, body thoroughly used up, totally worn out and screaming "WOO-HOO, what a ride!!!"