Search:

You probably cannot use a 6" wheel on that grinder. You would need to be able to shorten the distance between the wheels by a couple of inches to use the same belt length.

Regarding the difference...

You can use a hole saw mounted in a drill press to cut out various sized wheels but you would still need to true them up after the cut. They are not perfectly round, but very close.

106200...

What make & model grinder do you have? Send a link if possible.
Would it be possible to take off the wheel on your grinder that is crowned ( peaked) and put it into a electric hand drill, turn on...

Polyurethane Wheels | Drive Wheel | Polyurethane Products

This should be what you need. Lots of options available.