I've got a Yam 3.5 air cooled. all I do is change the plug replace the oil in box by the prop check for any leaks.
change the fuel filter check the fuel lines for cracks, I normaly spray "carb cleaner" into the carb.
Other than giving it a good going over looking for coroded wiring not much else I can think of now
Best of luck
Rog
I had a Mariner 6 which I think is the same block[Could be wrong]. Winter laying up is the key . Pressure wash down with fresh water. Teaspoon full of oil down the plug holes and at least one large tin of WD40 over the head unit, leg and wiring. In the spring change the leg oil, replace the plug, wipe down, check for rough wiring and pull the cord [fresh fuel]-should go
Ive got one as a spare engine incase my newer water cooled version lets me down.
Re maintenance not much to do.Top up or replace gear box oil,new plug, new shear pin ,new split pin through prop nut .Wash down and wd 40 every where is all my engines get.
A new set of points once in a blue moon may be needed ,if it aint broke dont fix it.