I would definitely start at the battery.
A long sitting battery can loose all its power and pretty much be worthless, and not recharge, or hold a charge, or have/hold any real cranking amps, in a little as 4-6 months. Don't use a car battery to try and start it, as there are too many amps.
Even a dead motorcycle battery on a trickle charger, will not provide enough juice to start the bike. If you have a buddy with a motorcycle that you could jump off of, that would be best.
And, a new battery is something your gonna need anyway, if that ones been sitting for a long time. I use the new Li-Po batteries, this battery has been AWESOME!!! https://www.amazon.com/Battery-Tende...tender+battery
It will state that it won't fit/work for our bikes, but i can confirm, as well as a ton of other motorcyclists, that these batteries are the best. be sure and buy a charger to go with this battery. Same manufacture sells a trickle charger. You'll just need to use the foam inserts so it doesn't flop around in the battery bay.
i wouldn't touch or look at anything else, until you get a new battery in there.