Snapper Pulley-Idler 4 Order part 1757823YP.
Inner Diameter: 2/3"
Outer Diameter: 4-5/8"
Note that Murray - Snapper - Simplicity are owned by Briggs and Stratton. Packaging may have Briggs lettering, but this is an OEM part.
See part number 1757823YP in the Snapper parts diagram, click here