Freewheels and cogs are two different things.
A freewheel allows you to coast. A track cog is for riding fixed gear.
You need a 3/32 20t freewheel. There are many available.
A decent, removable one which should go nicely with your H+Son wheels is at
https://www.modernbike.com/acs-paws-...3-32-gun-metal