Centering the sheave is not too hard but takes patients.
With a tape measure center the sheave as best you can and mount the rear wheel on the bike with rear belt off.
A dial indicator tool would be helpful. See Pic. If ya don't have one you can mark a stick and tape it to a fixed spot on the frame. As you turn the wheel the sheave edge (where the belt goes) should be at the marked spot on the stick. Loosen or tighten the sheave mounting bolts as you go and tap the high or low spots with a rubber mallet to center the sheave. This method will get the sheave very close. Next with you indicator stick still taped to the frame put the rear belt on and take the front primary belt off so the wheel spins freely. You'll be ably to see the clutch spring bouncing where the sheave is still not centered perfect. Now you can fine tune the sheave using the rubber mallet like before. Good Luck it takes patients. When a Whizzer mechanic installed the sheave he had a special alignment tool that he just slapped on and it aligned the sheave for him You'll get it centered, it might take a hour. It is important to have a almost perfectly centered sheave. Because the out of roundness will put uneven belt pressure on the clutch and crank and can cause serious problems down the road.
Pic of a Dial Indicator tool