What's a good place for replacement AW cogs? I've heard that the stock one isn't ideal, so I could kill two birds with one stone getting a 3/32 cog with a different tooth count as well.

I'm not entirely sure how all the different factors affect each other. The bike will have a 42t chainring and 700C wheels. What would be a good size cog to go with those?

P.S. I apologize to the OP for sort of hi-jacking this thread. I hope my questions help in your quest to build a 3 speed.