Originally Posted by Andy_K

If you don't want a flip flop hub, you can reuse the existing crank and bottom bracket, but you'd need to respace and redish the rear wheel to fix the chainline. I think someone already posted a link to Sheldon Brown's site where this is explained. You'll still need new cables and pads if you're planning to use brakes (which I recommend). I would expect you'll also need a new chain. It's possible you could reuse the existing wheels if you don't want to stop in the rain.