I've made paper bushings for small motor shafts when a lot of shaft torque isn't required. Cut a strip of paper that is a little wider than the motor shaft. Tightly wrap the paper strip around the shaft, then try to press on the encoder wheel. If it is too tight, snip off a little off the end of the paper strip, rewrap, and try the fit again. Keep snipping until you get a snug fit on the shaft. Pull the the paper with the wheel on it out just a little so the paper bushing doesn't rub on the motor housing. A little trimming and maybe a drop of crazy glue should hold the wheel in place.