[kevin.li.rit]

If it's not by much I would use a sprocket to step up the speed. I can't think of a good way to modify those gearboxes for that purpose. You can open up the gearbox if you want to take a look inside.

I would also try http://banebots.com/pc/P60K-nnnn/P60K-44-0004 or picking another one from andymark.