Hello Grześ,
welcome to Stridaforum! 
Seems that the Strida lightweight saddle is sold out pretty everywhere, just Bill has one demo model in stock:
https://stridacanada.ca/product/strida-light-weight-saddle/
Unfortunately, I’ve never heard of lighter cranks…principially you could use any matching bicycle cranks but I believe that the 4 bolt spider in diameter 104 mm is not that common.
To save weight here I’d recommend to switch back to the plastic beltwheel ( ~350 g) - these are much lighter than Aluminium ones ( ~750 g).
(Only at the two speed drives it is possible to change the cranks with, for example, “Tune” https://tune.de/ cranks.)
Cheers,
Chris