Plugs will probably help the starting some (Champion L77JC4) I'd probably just siphon out the old fuel depending on how much is in the tank and use it for a bon fire or something. Then fill it up with fresh.
You probably want to decarb the motor with BRP (OMC) Motor Tuner (I'm a fan of this.) Go to a ERude/Johnny dealer and get a can of it and follow the directions on the can. I bet it's never been done before. Oh and wait till after you do it to put the new plugs in.
To explain what that it is: Its a spray can of stuff that you spray in the carb throats and let it sit like 12 hours and then run the crap out of it. It removes the carbon deposits built up on the inside of the motor. Good for it and easy to do, like I said follow directions on the can. Should do it every year. Do not do it on a driveway or in front of house, garage door, car, concrete driveway... anything important. It will blow out a lot of black crap
I'd definitely change the water pump impeller especially if its been sitting and it sounds like it has. It's made of rubber and it'll dry rot. Change lower unit oil If oil shows signs of water in it (white color) you'll want to change the seals.
If plugs don't make a huge difference you might have to rebuild the carbs. Not hard just get the kit from the dealer, a Seloc Manual, and do it.
Welcome to the world of boat ownership/maintenance:D