Hello Ben,
thanks, also Happy New Year and welcome at Stridaforum!
Correct, the ancient versions had two plastic parts which snapped together, like, let’s say, “ball”(or more disc) and “pan” - just look below.
No (original), but 3D printable →
https://grabcad.com/mark.sanders-1
Even the later version is nowadays available from Korea only.
Don’t try them - they are too large for your MK1!
(I have seen once MK1 and Strida 3 brake assemblies next to each other.)
No, I guess the preowner was creative 
Or did you mean if you could use a standard cable adjusting screw to replace the shown, rusted one?
In that case I’d reply with yes, I think this is a standard part, too.
As far as I know there is nothing special at the cables, that is standard material which should be in stock at any workstore.
Btw, at this page, all the info about the ancient Strida versions is still present:
https://issuu.com/mark77a
Best regards,
Chris