Bob, you can try lightening the variator rollers, this may give you better acceleration ( but if you lighten too much it will be at the expense of top end speed)
I've put a thread up in the zx50 area that has info on carb mixture settings, you may be able to lean your mixture out a little and gain a few revs ( and more mph ) if the belt is taking all the ink from the variator, then your boss length is correct, an upgraded variator is the next logical upgrade.
For price though, try altering your mixture, be sure to fit a new spark plug though so that you can check its colour after a couple of days to make sure it's not leaned out too much