Good planning is the secret. A set of new tires will last you from London to Singapore, or from London to Capetown, or from the US to Venezuela, and from there to Buenos Aires or Santiago. So forget about putting 21" on the front, unless you want to do extensive Sahara touring. Michelin Desserts only come in 21" front, but you can improvise, use a 19" rear and shave it down to fit between the fork. I advise against carrying a spare. It's a pain in the butt loading and unloading it every day. If you're really in bad trouble, e.g. total write-off, you can have one shipped to anywhere in the world by DHL, expensive, but at least possible. Have fun with the logistics!