The link shows another example of overthinking- a foldable MB with a 49cc and top speed of 28, which indeed would be very rapid for such a thing.
I think the future really is thinking abut less power less noise and less speed and more practicality for shorter urban distances.
I'm in the process of building up an old 80's Schwinn World Sport frame with some stingray bars and a 5 or 6 rear derailleur only. I'd put a motor on it if I could find a smaller one, but I've already got a 49 build, and don't need to duplicate the experience.
Got this frame for $10 on craigslist, I'd love to put a smaller China Girl style motor on it I could pedal up to comfortable speed- an easy good roll up to 10 or 12 mph, and then clutch on and kill off at will, putter along at the legal 25 mph which is plenty for someone 62 who really does not want to fall off now. Maybe someone will make one happen someday. Our 50 and 66cc motors were both made to push HEAVY cruisers with outdated technology. The bike world has moved on in it's own way 700c wheels and tires are available in a wide variety of rim and tire widths to suit everyone's needs and tastes. I like ones that will both roll and still wear and carry the weight my several baskets can take along. My 50 cruiser has widths of 27mm or 1 1/8 wide. They don't need 50 ccs to do 25 , once the bike is rolling at least. Such a motor on this frame with a lighter tank- no tensioner, alloy motor sprock and light Industrial Trike 415 chain, would weigh in at about 37 or 38 pounds- a bike easily peddled in parking lots or through the park.