I know what you mean; these machines have hair-triggers, for sure.
The pulley on my motor mounts onto a shaft via an allen screw ... yours should do the same, no? Just measure the diameter of the shaft and any decent hardware store should be able to set you up with a small pulley -- get the *smallest* one you can!
My machine does run slower because of the small pulley, but not *that* much; it still flies! The smaller pulley gives me a whole lot more gradual speed between the "giddy-up" and the "whoa!" stages.