I routinely fix bikes such as this every week. It's nothing that some time, some dough, elbow grease, diet coke, and some Jimi Hendrix can't fix. You can google or youtube articles on chain replacement, tube replacement, and cable replacment and with your ambition you could do it yourself easily. The rusty screws would either need replacement or resurrection with a wire wheel on a drill. Sometimes other parts just need a good cleaning with some spray lube du jour and a toothbrush to make them both presentable and funtionally efficient. Good luck!