Am thinking as I type out a kludge - get an old SunTour 5-speed freewheel with cogs you want on it. Pull the cogs off. Put the two you want back on with enough spacers between that the 1/8" chain works.

If I recall correctly (it's been decades since I took either a Winner or Perfect FW apart) the bottom (larger) 3 cogs fit on the larger diameter splines and would be the ones that line up nicely for your use in single speed mode so find a FW with small (racing) cogs that are not too worn. That could be a bit of a challenge.

Again, this has has about 30 seconds of processing, so proceed accordingly.

Ben