you can buy a small socket tool for a few bucks at any local bike shop, and using a big wrench, remove the sprocket (they're tough to get off, but with force, you'll get it, here's a video showing how ) Go find another junker bike (or buy a new sprocket) that's the same gearing, and using the same socket tool, put the new one on, and you'll be good as new. Depending on how resourceful you are, you can get that fixed up for next to nothing. The rear wheel is still perfectly fine, those sprockets come right off so you can put a new one on, and you'll be good to go.
