Whoah! That is one eye-catching bike! Was it already that color, or is it a repaint? Also, which brand/model of fenders are those? They're like nothing I've ever seen.
For the "free" side of the flip-flop hub, you need to purchase a singlespeed freewheel. You don't buy a separate cog, just the freewheel in the size you want. And you don't actually need to buy the freewheel, either. You can just leave that side of the hub alone, save $25 that you don't reeaaallly need to spend on a freewheel
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That'll be one sharp bike when it's done!