Well recently at halfords i broke a kiddies bike so i had to take the back wheel off, only to notice the fixed cog(usually on 12" bikes) can screw off :ermm:
They cum in loadsa sizes, ive seen 12-16-18-19-20-21
Go to your local tip, find a kiddes bike steal the wheel, rip i apart jam the hub ina vice and wack a chain wip around the cog :huh:
Saved your self £15-20 ^_^