"How does the guy going downhill on a skateboard at those speeds stop safely?" Safely? According to 'longboard fails', oftentimes they don't. This looks wise. Safety eqpt? What's that. One Road Rash, Comin' Up. https://www.wikihow.com/Ride-Downhill-on-a-Skateboard
Going to build a Triumph cafe racer , looking now to buy a donor bike with a 900 triple . Something along these lines ..