draco, the stock airfilter from the dealer is $7... But it's totally not necessary to buy a new one.... I have over 18,000 miles on my stock air filter, and still going strong...
What you can do, is wash it... Seriously. Go to your dealer, and get some foam air filter cleaning solution, and oil. Then wash the filter as per prescribed in the directions on the bottle, and re-oil it with the foam air filter oil... Good as new! You'll be surprised how much gunk comes out of there!
Let's see here, along with the same air filter, I've had the air filter cleaning kit since before I bought the bike, and it will definitely outlast the bike's airfilter. So for $14 bucks, you're good to go on the life of the bike...

And even if you got a new airfilter, you'd still have to oil it, so might as well get the right stuff. Mine is Kawi brand, it came in a kit...
And change you're oil. I don't know about gas additive to evaporate water out of fuel, but you could just drain the tank and fill it up with new, high grade gas and see if it works.
Concerning the spark plugs, yes, two rubber stoppers on the spark plugs, that go WAY down deep into the heads.... Pull those out, and then stick in a spark socket on an extension to get your plugs out... makes sense?
Did I miss anything?