1. Lace a rim to your hub, it will save you a fortune and lots of time matching up the bearings, spacers and brake system.
2. Because 21 inch rims usually use much smaller tires, the difference in total diameter will be less than you might think. The biggest thing is going to be finding a fender that looks right and is easy to adapt to your forks. You could do without a fender but trust me, that gets really nasty if you get caught out in the rain.
