Ours can do about 45 mph on the open flat, but they can also climb a pretty mean slope out in the woods. We could make them go faster, but we really like the well rounded race/offroad profile. The frames are tough enough to take at least mild jumps (although, with no suspension, we tend to avoid that).
We don't do any mods to the engines at all, just regular stock twin cylinder briggs engines. Stretch posted a pretty thorough step by step of how we built them on another forum if you are interested.
http://atltf.go-board.com/t1074-craftsm ... -38-builds
That forum is unfortunately populated mostly by teens, but it was the closest thing we could find to what we are doing with our tractors.