If you really want to break that 40 mph barrier, you can try using some of the 50cc KTM engines. They're small enough and built a lot like the bicycle engine kits.
These engines used with heavy duty chains will allow you to take straight off without pedaling, but they're costly. You'll also have to take it to a welder since they have no mounts for a bicycle
Also for the bicycles to actually handle that kind of speed and vibration, you'll need a real expensive mountain bike that's real heavy duty built like these
they're real heavy duty bikes built a lot like motorcycles to handle the vibrations from rough terrains. But they're like 1000~2000 dollars, so that combined with the ktm engine and other parts plus the fee that you're gonna have to pay at a machine shop to get it welded and such, you're looking at 3000~3500 dollars. Which will probably get you a used ninja 250 that you can ride on freeways
In conclusion, set up that you have right now is really good. Just ride on sidewalks and bike lanes and it'll get you around a lot faster than bus or even a car since you can bend a lot of traffic rules. So keep it like that it's a nice set up.
Oh and also, I know it gets scary riding real slow in the traffic, but it's actually a lot safer since the cars behind you won't take much effort to pass you instead of having to go up to 40mph to pass you