Two hints for a possible bikegang order:
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Caution; the goods come from Taiwan, depending on your local bureau for customs duty there may be additional costs.
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At the same page, a little bit hidden, you can find the link “this listing” - below specifications of the kit. Here you can order special tools; you need at least that for the kickshift buttons, perhaps a 60 mm hook key (if you can’t find it cheaper locally).
For more detailed information I’d highly recommend to download the original Schlumpf manual:
http://www.schlumpf.ch/hp/handbuecher/WHB.sd.engl.pdf
Installation and adjustment of the gear shift buttons - please study chapter B-5.
For maintenance go to chapter C-1 
The installation process was shown in this old thread:
Re: Has anyone retrofitted a schlumpf on their strida?