It's gonna be next to impossible to stretch that chain and get that front sprocket back on, if that is indeed an original or old chain, then I suggest changing it since its off anyway. I'm sure you'd rather put on a new chain than have it break and slap a hole in your case. Cause fixing that is really, really sh!tty! Changing the chain is pretty painless anyway, but if you must use the old one then I would suggest loosening the rear wheel axle and moving it as far forward on the swing arm as possible to see if you get enough slack on the chain to get it all on the front sprocket axle. Good luck man!!
Rick