sounds bizarre but I had a trail-a-bike in for repair (with single cog freewheel) where the kid stripped the freewheel threads off the hub.

I'd be looking for a decent hub for the project. Avoid those stamped sheet metal bso hubs. I've seen scores of failed hubs. Freewheel issues are almost always due to lack of any maintenance. Pawls getting stuck 'open' in -30F weather or rusty from being left out doors in the rain and snow.

and don't get a freewheel that has no remover slots. yeap, they make em like that.

For that creation, I don't think high speed is going to be an issue. You may need a 22t freewheel.