Re: Many questions, wanna make a purchase!
Your $800 scooter did fine because it was built as a scooter or a "motor driven cycle." These bikes that can keep up with the traffic and are allowed access to a full lane are designed to withstand vibrations. Even the cheapest and crappiest scooters will at least have some engineer work on safety and vibration issues or else it probably wouldn't have made it to US as a street legal vehicle to begin with.
And let me tell you something, I actually did try ALL performance upgrades possible on a chinese engine on my first 66cc build
I had 66CC flying horse motor, SBP jack shaft kit and 8 speed cassette, 11 tooth sprocket, CNS Racing Carburetor, SBP Expansion exhaust, iridium sparkplug. ALL of them on a 150 dollar Walmart bike. I did hit my dream speed of 45 mph on my eighth gear.
I made sure I welded up all these performance parts properly instead of using brackets and sillicons.
The result was everything about the engine and the motor kit was fine but all the torques and bolts were falling apart on the bicycle aspect of the vehicle. Later on the thin rear end of the bike frame that holds the wheel was almost about to snap. Not to mention one out of every two rides, the torque on my handle bar would get loose, so on short rides when I don't have my tools in my backpack, I had to pull off some crazy stuff to maneuver my bike with a wiggly handlebar. That's when I realized the whole thing was freaking dumb and reverted everything back to stock.
You said "there's going to be an idiot, trying to "tune" the thing to make it go like a rocket, and in the process overburden some parts." And anything over, and even slightly over 35 mph on a walmart bike IS "making it go like a rocket." 45 on a bicycle is CRAZY. Not to mention that they don't have the same aerodynamic designs of a motorcycle. So you're gonna have to fight the wind resistance crazy hard once you go even 1 mph over 35.
Even when i'm going about 20~30 mph on my mostly stock engine kit bike, I took the bike to the welding shop to weld reinforcements on the weaker part of the frame.
You said you're more interested in MPG then performance. But for MOTORIZED BICYCLE 40mph IS EXTREMELY high performance. And actually the low MPG is a package deal with the relatively low speed. Once you install all these performance parts to get your desired speed, you'll go through those half gallon/1 gallon tanks so fast.
Even at 25 mph, I still get to places on time and I make spandaxers eat my dust.
But anyways if you really really really wanna do this, go get a morini and a worksman bike.
Worksman bikes are practically indestructible. But at that point, you might as well buy another scooter with that money