Bike looks good.
When my belt starts to wear out I will probably do this conversion.
It has an ISO threading of 34.92mm?
Is the one you used for single speed road bikes or another type of BMX freewheel?
Can you tell us what brand/model freewheel you ended up using?
Yes the freewheel cog has an ISO threading. I don’t know the brand, it’s a bronze cog. Good luck and keep us posted ! 
great post. I hope this month i will done something similar… or even more.
I want that 65T (or 60T) chainring from Five Big, and with a 16T freewheel I will have a ratio of 4.06 (or 3.75)
I will see how easy will be that ratio to manage. And after that putting above all the 3 speed schlumpf gear system, i think it will become the fastest Strida? (I will call it Zuper Strida)
I know this is old but @ttakata73
the freewheel is YIH Industris, Honk Kong, and it is a standard 35mm (or 34.92 more exactly) freewheel (more used on Trial bikes, the inner circle just to be clear of the freewheel is 35 mm)
Schlumpf has just 2 gears…
yes 2 speed. Sorry for that.
No prob; just to be sure that you didn’t mix up the Schlumpf with the actual Sturmey Archer 3 speed EVO drive (which isn’t compatible to the single/dual speed frames with the ~ 60 mm Ø bottom bracket.) 
I mixed them up and of course I spent a few hours to check if it is possible to upgrade…
Hope it’s clear now
Maybe you should consider an ATS drive, it’ll be much cheaper than a Schlumpf.