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

Water Pump Impeller Disassembly/Assembly
Water Pump Housing DisassemblyParts Removal/Installation
WARNINGGasoline is extremely flammable and can be
explosive
under certain conditions. Turn the ignition
switch OFF. Do not smoke. Make sure the area is
well-ventilated and free from any source of flame
or sparks; this includes any appliance with a pilot
light.
NOTICE
...
External Shift Mechanism Removal
Remove:
Shift Lever (see Shift Pedal Removal)
Oil Pump Drive Gear (see Oil Pump Drive Gear Removal/
Installation in the Engine Lubrication System
chapter)
Circlip [A]
Washer [B]
Special Tool - Outside Circlip Pliers: 57001-144
Remove:
Shift Shaft Assembly [A]
Washer [B] ...
Gear Position Switch Removal
Remove:
Oil Pump (see Oil pump Removal in the Engine Lubrication
System chapter)
Engine Sprocket (see Engine Sprocket Removal in the
Final Drive chapter)
Fuel Tank (see Fuel Tank Removal in the Fuel System
(DFI) chapter)
Disconnect the connector [A].
Remove:
Screws [A]
...