SA still makes new cogs in a wide range of sizes and you can order one of those or if you have a bike co-op you might find one there... the stock cog is a 15 so you will need a longer chain and the chain length and dropout design on the Twenty really works better with an even / odd combination.
The dropout is short and is angled upwards so there is not a lot of room to play with chain tension... a 17 or 19 would be an easier switch.