What you do, really, is up to you. The Viscount line was infamous for having non-standard parts and sizes, so you're possibly going to run into some more issues with that bike than you would with many other makers. I had one, and had some issues with the bottom bracket and crankset, and decided to pass it on to someone who was a little more resourceful than me. I've got nothing against retrofitting an older bike with newer bits, but yours doesn't sound like it would be an easy transition.