In the early 1960's I had two Whizzer motorbikes (goggle it) which were based on a purpose-built 4-stroke engine designed to bolt (clamp) onto a paperboy bike. They were designed to provide cheap general street/road transportation. Still they had their mechanical weaknesses, resulting in thrown connecting rods despite several factory redesign efforts. That kit you are looking at is a Chinese sorta-copy of the Whizzer theme, but very scaled back. My bet it will be ready for the junk bin after a few hours of use. But try it and report back. BTW, the displacement (cc) is usually marked on the base of the cylinder on moped-type engines, and you should believe the cops would love to catch guys not complying with the very specific (and punitive) moped rules. Google your state's current moped laws - most states have tightened up the moped restrictions in the last year or two. Most do now allow 50cc "motor scooters" capable of less than 35 mph as mopeds even though they don't have pedals. A Chinese 50cc scooter might be a better investment than MTB plus a motor kit.

I've seen a few of those Chinese bike motor conversions in use and they were slow, noisy (think leaf blower) and one spontaneously threw its drive chain, apparently for my simple amusement.

I currently have a Columbia/Sachs moped built in 1978 and it has a German-built 50cc Sachs engine and has wheels, hubs, suspension, brakes, lights and horn like you would see on a small motorcycle. It is street-legal and licensed under our current state moped laws. It's still a little iffy to drive on the street because when cars approach from behind, they usually treat it as a bicycle, and force their way past it anytime and anywhere.