Hi flectalux,
principially you’re right, the Strida’s freewheel is in fact a proprietary part.
But not the thread, that is a common ISO thread with a diameter of 1,375" and 24 TPI.
That means, theoretically, that CDX freewheels should be available - for example this one:
A matching beltwheel should have 72 or 74 teeth to keep the original Strida gear ratio.
Here’s possibly the first problem, the highest amount of teeth I could find so far is just 60, that’s not good:
The next obvious problem will be that of the belt - as you know are belts available in certain sizes only and the “adjustment window” of the Strida’s excenter is not that big.
And the excenter becomes also important in the case that you need a completely new crankset!
(You noticed the beltwheels five - arm spider; just the Strida has four arms.)
That is doable, but only by using a modified ATS excenter (like that which I made for the efneo drive).