- Correct would be Suisse engineering, btw…
From here:
Re: Has anyone retrofitted a schlumpf on their strida?
If you mean your first two links:
They seem to be complete, but without tools.
The third link is a longer version which is not suitable for Strida (no beltwheel, no belt, no eccentric bottom bracket).
I’ve done a lot of work just to help Strida riders to judge about this delicate and personal question.
Re: Has anyone retrofitted a schlumpf on their strida?
Nobody else than YOU can tell if it will be complicated for YOU.
An experienced shop must be able to do; they will already own most of the required tools.
Just a short, not productive mail contact with bikegang.
My opinion telegram style:
One gear - much too less.
Two gears, heel gear shift - cool, but still too less.
Three gears, backwards pedaling gear shift - not my style, well, Ok…
Seven gears - now we’re talking 
Shifting by heel kick during pedaling forwards. 