sshakari

Building my first singlespeed bike and I am not really understanding fixed/freewheel rear hub. Here is the story:

- got Origin8 fixed/freewheel rear hub wheels
- my crankset 48t 3/32 and so is the chain is 3/32
- want to ride 48x20 freewheel

Having a tough time to find a 20t cog and freewheel with 3/32.

What are my choices?
Would buying a 1/8 20t freewheel cog work with this combo?
Is there a way to get a freewheel "body" and buy a 20t track cog?

thanks in advance.