Nice mini bike, but I'd have been crazy to ride 50 mph on one of those gas ones back in 1970. Same problem as then, kind of hard to even stay on the thing if you hit a big bump on it at 30 mph or more. Needs at least a primitive shock, and preferably motorcycle grade or high end MTB suspension to ride that fast on anything but perfect road.
I have run 50 mph on an e bike with no suspension at all, but that was on a baby butt smooth racetrack. Once on a practice run near my house, I did not see a big hump for a manhole cover in time. At 50 mph, I was riding on the front wheel only, back wheel a foot off the ground. That was quite a thrill..
You looking for that kind of thrill? Follow Mad Rhino's advice here I say, he rides hard and fast, and spends the money to have a base bike that can handle motorcycle like speeds, on any terrain.
These days, even used, you are looking at a lot of money just for decent front forks. Cheap stuff, MTB bikes that sell for under 500 bucks, might do for on the street, 40 mph, and moderately hard dirt at 25mph. 40 mph club is actually pretty affordable, but 50 mph club gets pricy fast, just for big motors, big batteries, big controllers. etc. let alone a bike you'd want to ride that fast.