I've never heard of any of these buggies having a speed restrictor of any kind. 40mph is a typical top speed for a 150cc buggy. You could make it faster by changing the sprockets but it will ruin your low end power. If you need more hill climbing power look into doing the CVT mods (rollers and torque spring) and opening up the intake (redneck uni mod is cheap and easy) and exhaust.
Jamie _________________ Twister Hammerhead 150SS
9 gram rollers, 2004 gear reduction, Uni Filter w/Redneck intake, Gutted Exhaust, Bando HP Coil, Sandman/Blister reverse bracket