Here is an online auction in Taiwan which is selling some kind of “add-on engine” for bicycles.
tw.f5.page.bid.yahoo.com/tw/auction/e28039155
Interestingly, a photoshopped picture is found on the auction. 
Whoa - Fun 
I remember riding a Solex as a 13 yr old kid in France – my 1st motorbike ride !
Happy Memories
The Snag on a Strida - is it stops the wheels clipping together.
But, A tiny powerful (500W +) brushless motor from an electric RC plane would give about the same power and could be fitted in the same space as the front axle mounting (ie invisible), with batteries in a line, inside bottom tube - you’d have a Super Stealth Strida (SSS not SLO). All these parts come from China so it would be interesting to see if a fake maker could add it all up and innovate - rather than just copy. ???
