There is a couple ways to do this without a lot of fuss. First the big pulley is more than twice the diameter of the little pulley and they have different shaft sizes. If you want a single speed then decide if it currently goes twice as fast as you want it or f you just want to slow it down by say 30% .. If you want to slow it by about half then double the size of the small pulley. If you find a pulley with the right shaft diameter that is half the size of the big pulley then do that. If you can find some pulleys then you have two choices. Most guy would buy a couple or at least one step pulley with 3, or four steps in the right direction... say 3 steps with the largest step being a little smaller than the large pulley on your motor because it is easy to move the motor to align the pulleys but really hard to change the location of your small pulley. Does this make sense to you? If not then look 0nline at the sites for build your own grinder or no weld grinders and you will see setups with step pulleys that work very well to change your belt speed. If you can buy used pulleys or scrounge them try to get cast iron pulleys...they are much better and cause fewer problems. I hope this helps you.
plus it is technically my old mans.