There's a few vids on you tube that may give you a better idea of the procedure ( at least you don't have any gear linkage to worry about )
but briefly I would empty the fuel tank ,
Tip the engine upside down makes it easier on a 2 stroke engine that size ,
From memory remove 2 bolts the gearbox & waterpump should now separate & pull out from the from the leg ,
Where the drive shaft goes into the gear box that is the water pump with the rubber impeller inside
Undo the bolts & slide the pump housing/ casing off the drive shaft
( carefull you don't loose the small ( half moon woodruff key or if I remember it could be a small dowl or pin that goes in the slot on the impeller )
if the impeller happens to slide up with the housing ,
You will now see the rubber impeller which slides off the drive shaft .
Assembly is reverse procedure ,
I usually clean up the bolts / threads & put a wipe of waterproof grease & on the top splines on the drive shaft
but make sure the woodruff key or dowl is fitted in its slot or its not worn or rusty with corrosion ( (there not all stainless) otherwise the impeller wont turn and pump when you locate the drive shaft back in the leg &
& make sure the water pipe locates proper in its hole when you fit it back to the engine
You may also have to turn the prop back and forth slightly so the drive shaft locates back in the engine splines before it will fit back before tightening .