Knob,​ Selector Spring

1) Raised and supported the center of the lawnmower frame, freeing the wheels from the surface of the floor. ( I used a 1 gal. paint can, which provided a wide surface, and weight to hold the mower from moving. Didn't have to block the back wheels.)
2) Used a screwdriver to help remove the plastic wheel covers (pop off hub caps) and exposed the nut on the wheel, and used a socket set to remove the nut from the wheel axle.
3) Removed the wheel, and the washer between the wheel and the inside of the broken shield, and cleaned the crud from the washer surfaces.
4) Cleaned the crud from the E-Ring, used the straight edged screwdriver blade and the small hammer to tap the E-Ring from the pinion gear axle. Then removed the old pinion gear and corroded parts, and finially the broken plastic inside wheel cover behind the pinion gear.
5) Cleaned and removed all of the crud and rust from the wheel axle as well as the pinion gear area of the drive axle, then lubricated the axle parts with WD-40 to help in the reassembly process. Carefully cleaned the crud from the small E-Ring groove on the drive axle to enable the new E-Ring to fit and lock the new pinion gear assembly in position.
6) Installed the new plastic Pinion Gear shield over the two axles, making sure to work it back over the drive axle hub, then installed the new pinion gear pieces on the drive axle and finally the new E-Ring to hold that in place.
7) Installed the washer over the Wheel axle, then installed the new wheel, meshing the inside teeth on the drive wheel to the pinion gear teeth. Reinstalled the flanged wheel nut, and tightened it, then reinstalled the plastic hub cap.
(Note that there is a "You Tube" Video covering the wheel replacement process that might give some confidence to a novice attempting these repairs)