I would just pick up a different motor, as fractional horsepower motors are quite cheap, especially at swap meets and garage sales. I mounted a wire wheel to a 1725 RPM motor, as most of the time I like the slower speed better.
Seas pulley idea works, kinda like the brake lathe set up... or what Tom said get a different grinder... both suggestions prob beat rigging what u have and poss creating a fire hazzard
I actually have one somewhere. The ones I thought were pretty interesting were the hand crank grinders for sickle bars with about 4 sets of step up gears.