Gas tank is from a 70's moped. It looks like it's held on with zip ties. To mount it right you'll need to weld some mounts to the frame.
Now for some criticism. Try not to get offended.
I wouldn't call this a "restoration." What you have is a low cost mountain bike with a bunch of parts haphazardly rigged on with electrical tape and zipties. If I was given this bike, I'd take off the motor, tank, and the forks and throw the rest in the trash.
In my opinion, it's not worth rebuilding. I think you'd be much better off starting fresh than trying to work around all the problems this bike has. The seat is, well, besides being barely attached, doesn't... won't... I wouldn't... look, it's crap, ok? Don't know how else to say "you'll end up straddling the tire while your precious parts have all their skin removed.
Back wheel, chain, sprocket are all rust. Rust looks good on a 70 year old bike. Not a 90's junker with a cheap motor kit.
Speaking of the motor, if it doesn't run, take off the pipe, (I can see rust on the bend in front, that might be crappy, too) and save it all for parts.
Buy a new kit for $169 and junk the rest. Why fix a broken plastic throttle? Trash it.
I could go on, but you probably get my drift. Which is, leave this one by the curb, buy a bike that hasn't been butchered already, buy a new motor kit, and save yourself a ton of headaches, and probably some money...