13t, 14t, and 15t freewheels exist, but they are metric threaded. You need to build a wheel with a BMX (race, not freestyle) flip flop hub and space the axle out (and possibly replace the axle) to accomodate your frame. Cheap BMX hubs are readily available.

Or, if you want a really interesting wheel project, build a wheel with a BMX cassette disc hub. Use a bolt-on cog on the fixed side and standard freehub cogs on the freewheel side, which go down to 11t.