Continued...
Frame reliability is also a huge consideration. As Jerry pointed out, there are bikes that can and have gone faster than 60mph. They aren't $200 machines, nor do they have the added weight and vibration of an engine.
Wheel bearings were also a great point that was touched on. I wouldn't trust a cup and cone bearing beyond 25 to 30mph. They aren't meant for that kind of loading, so you would want a wheel with roller bearings, at a minimum. I would also want thicker axles.
Some of this stuff.does exist, like Gemini wheels solving the last 2 points, but you are again talking about 400 bucks for wheels. Well worth it, to be sure, but not cheap.
A couple thousand dollars later you have a fast bicycle that may be reliable. But will still likely need a lot of maintenance to keep it going, and always a potential safety risk when pushing it to 60mph.
I would just buy a motorcycle at that point. That's literally what I did too. I bought my Hawk 250 in 2017 to replace my former MB because I wanted something that could safely, and reliably, do 60mph. I still have it, and I've been tinkering with it so much that basically every part of the bike has been upgraded or modified, and it now does 75mph, and accelerates way faster than it did stock with a much taller geared sprocket setup.
