My KX500 was my first ever dirtbike. I didn't really need a power reducer. Fear of death handled most of it...
If one really wanted to make a limiter of some sort though, why not put something like cable insulation etc... around the bare throttle cable inside the carburetor so it stops the slide from opening beyond a certain point? Need a lil more speed? Unscrew the slide cap off, trim off some material, there you go.