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Best off getting a workshop manual for them, will walk through procedure and what not to do.
If they're still in warranty, don't touch them, keep it dealer service until warranty expires.
If you're running a twin rig, your fuel and electrics for each engine should be totally seperate, this will eliminate any common problem
With regards to what you can do? Keep fresh fuel in the tanks, use a quality oil (yamalube in your case) and get a water seperating filter if you don't all ready, check and drain it reguarly. Won't hurt to periodically spray the power head and leg with inox or similar,
first step, is get a proper OEM Manual, not a copy or a no name one, they are fairly simple Motors and will give you years of use, easy home hanyman maintenance is, gear oil, plugs, general greasing/lube, removing prop now and then to check for fishing line and stuff that can wreck the shaft seal, after that when you are a bit more familiar (and you have your manual) then a water pump and thermostats and so on are (depending on your use) about 2 yearly replacement/checks
Mate good on you for taking an intrest in your motors, if for safety if nothing else.
Sure you will save some coin on the way.
Thing is your going to have to spend some money first, I would recomend tooling up a bit, and then investing in some parts.
With the tools, battery charger, compression meter, torque wrench, desent set of sockets and spanners, multi meter, test light.
For spares water pump kits x 2 thermostats x two, gaskett paper for obvious reasons spark plugs, Oil drain plugs seals Etc.
Then your going to need Oils, sealers, scrapers, razors hole punch kit.
As you can see the list will go on! and on ! and wil for many years, and theres still sh!t I could use but don't really need, but all the same will probably get it even if I don't use it.
But if you have no real intrest in mechanics or quality tools and a clean garage to do it, then save your money and keep using a mechanic. Cause you and your family will be better off. Honestly playing with boat motors is probably not the safest place to start practicing on motors, try lawn mowers, cars M/cycles first.
If something goes wrong at sea it will be more disasterous than the lawn mower in the back yard.