I've read about modified 90s doing 40 - 45 (and more) MPH. For some of the super racers, wallet depth is their only limit. Perhaps the sound barrier is the real limit???? On the more serious side.... Do consider the ground conditions for these faster speeds. When its suspension is in pure strock form, the risk of breaking a Swing Arm, A-Arm and/or snapping a frame dramatically rises when one exceeds default 32mph limit &/or pushes past minor trail riding conditions at slower speeds. If you plan to exceed 30-PLUS mph, seriously consider a stronger swing arm, wider A-Arms, applying gussets to the frame and installing much better shocks. Thus, allowing your young rider to "hang onto it" when hitting the throttle.
Talk about "souping up" a 90, don't forget to add Billy Holt at WRH Racing to your performance parts research list as well. I hear he as some great performance parts that would easily exceed the default 32mph limits. His contact details are: - Press Here -
If wondering, we're using a few of his parts in my son's Trail Pred-90. His parts and tweaking advise has been "right on" every time.
Hope this helps as well....