If the surface is corroded or if the blades are damaged, replace the impeller

Water Pump Impeller Disassembly/Assembly
Water Pump Housing DisassemblyEngine Sprocket Installation
Replace the sprocket washer and axle cotter pin.
Install the engine sprocket onto the output shaft.
Apply molybdenum disulfide oil solution to the threads
and seating surface of the engine sprocket nut.
Tighten:
Torque - Engine Sprocket Nut: 145 N·m (14.8 kgf·m, 107
ft·lb)
NOTE
...
Engine Stop Switch Operation Inspection
First Step
Set the gear position in the neutral position.
Turn the ignition switch to ON.
Turn the engine stop switch to stop position [A].
Push the starter button.
The engine does not start.
If the engine starts, inspect the engine stop switch (see
Switch Inspection in the Electri ...
Shift Fork/Gear Groove Wear Inspection
Measure the thickness of the shift fork ears [A], and measure
the width [B] of the gear grooves.
If the thickness of a shift fork ear is less than the service
limit, the shift fork must be replaced.
Shift Fork Ear Thickness
Standard: 5.7 6.0 mm (0.224
0.236 in.)
Service Limit: 5.6 mm ...