Hi.
Had a similar experience after leaving my bike unchecked in my own garage for the better part of... 18 months...
1. Ditch the old battery, it's hopeless...
2. Drain thegas (you've done it, ok) you can recycle it in your car
just remember to top to "full" at the gas station, you don't know what chemically happened to the old gas...
3. Get a can of WD40 or similar, spray a looong dose (10 secs or so) in the cylinder through the spark plug hole (taking the tank off will help)
Leave it to soak in overnight. This is to help the piston unglue itself from the cylinder walls.
Good for the piston rings too
AND helps when starting for the very first time.
4. I advise against using a car battery for the bike's electrical system, it has such a huge surge current you could fry wires right through,
whereas a small (moped or similar) 12V battery will eventually only fry the fuse
5. BEFORE trying out the starter motor, pull the spark plug, engage 2nd gear and push/pull the bike back and forth a few feet;
you should hear the typical "chuffchuff" sound as the air is sucked in and pushed out of the cylinder.
That is good.
BUT if you hear any scraping - squeaking sounds...
... aren't you glad you were merely pushing the bike rather than starting it ?
6. Once all these tests have proven OK; only after you've renewed those few inches of fuel line with new rubber;
only after spark plug and solenoid have shown to produce a nice string spark;
pour a quart or so of fuel, turn petcock to "prime", give it 15" to fill the carburetor bowl, pull the starting knob and give her a try.
She may stutter and die out, just avoid gunning the throttle, she should start.
Best of luck.
PS Once it's "all systems go" may I suggest you replace the stock petcock with a Raptor.