It's a nice bike, but you will likely run into compatibility issues if you start trying to swap things out. Standards have simply changed since this was sold. I would follow others' advice to swap out the derailleurs to something cheap from Suntour to keep it functional, but otherwise just leave it as is. This kind of bike is perfect to start tinkering if you want to get into it, but I would limit that to a tear down and rebuild, not pursuing a bunch of changes. Preserve its charm, keep it alive!