Wash outside with soapy water.
Flush engine with garden hose - warn the neighbours before starting !
Disconnect fule hose and run the engine till it stops. When it starts to cough and is about to stall squirt Quicksilver Storage seal in the carbs to protect the cilinders.
Grease moving parts on the engine
Spray engine with WD 40
Change gear oil
Have mechanic change impeller every 2 yrs.
When greasing use a gun on all the grease nipples and especially grease the thru-tilt-tube steering.. Also I wouldn't bother changing the water pump impeller until the earliest of 150hours in clean water, 4 years, running it through sand.