If you know someone with a 3D printer and only need the cog:
http://www.thingiverse.com/thing:530961
Some libraries have them. The one I went to loads the file into the software and then tells you how long it will take, and charges according to the time estimate. (They wanted $30 for the item which made it pointless. But if you only need the cog, I think it's quite a bit smaller than the item I wanted. A least two-thirds smaller.)
Use isopropyl alcohol (BigW for e.g.) to clean before putting a new one in - as above - contamination. Or metho will do if you have nothing else. With cotton buds, tissues, etc. (I've read metho contains a slight amount of some kind of oil, but I've never seen any evidence of that and I've use it on lots of things from electronics to glass.)
Also it's one tooth out, apparently it doesn't make a lot difference, since the speedo are rarely accurate anyway. Oh, and if it stops working, you can download HUD speedometer apps for your phone until you fix it.
