Dude, that is a loooot of corrosion. Like mopedallday said, your carb is toast. In addition the the chem neutralization with evaporust, you're going to need to mechanically remove the debris. Put some bolts and nuts in the tank and shake all that crap loose so you can flush it out.
I'd be interested to see closer pics of the fairings and body. Is there any rot in the plastics or frame?
Regarding the piston being seized, WD40 might not cut it. PB plaster or another heavy duty penetrating/creeping oil might help. WD is more for water displacement (ergo "WD") and cleaning but it does a lot of stuff well enough. Best luck I've ever had was using diesel fuel (old dodge truck motor) but it took a loooong time. Also steel and cast iron so the chemistry might be different.
Good Luck!!!! Interested to see how this developes for you. (edited)