Hello Paulo,
welcome at Stridaforum!
Wow, thanks for the impressive introduction and for teaching,
never before heard of ZAMAK!
I guess they’ve used that back then because it was easy to produce and lighter than steel,
who cares about durability…
But you need not to wonder, many synthetic materials become brittle by the time and some simply collapse 
According to that Old stuff about the past generations of Strida
must the plastic parts of your bike at least be ~ 22 years old, most likely more…
However, the only thing I can suggest is to get in touch with other MK1 enthusiasts.
I do know also that in Vienna must be at least one MK1 (in running condition!), sadly I’ve never seen it in reality…
inseq’s MK1
Good luck!