I ran flathead Briggs engines with their governors bypassed, just wide open on various go-karts, and every 3 hp push mower I ever had as a kid had a piece of wire on the throttle plate so I could momentarily hold it wide open through thick grass. Never blew one up.
Except my friend had a flathead 5 hp on a go kart that threw a rod. But they were running it without the shroud. They'd put the shroud on just to use the recoil starter, then pull it off because they lost the bolts. The shroud directs wind from the fan blades on the flywheel to make it blow across the cylinder fins. So it overheated. The rod went through the engine block and got lodged in the temporary seat backrest I made out of a 2x12 after the original plastic one broke off in a crash.
They don't make flathead engines any more that I'm aware of. Everything is OHV, made by Honda or copied from Honda.
Same 3600 RPM governor but they make racing flywheels for them that are designed for 9000 RPM.
To get 9000 RPM out of one though, you need to rebuild the whole thing though. Motorcycle style slide carburetor, tighter valve springs, physically remove the governor parts inside the engine, and a billet connecting rod I think.
I don't plan to do that. There's no reason to. Why would I want to get 15 hp out of a 7 hp engine, when I can just go buy a 15 hp engine for less than I'd spend building up a 7 hp?
Of course, a 15 hp won't fit in my minibike, but I don't want it to go any faster than it already goes anyway.
45 mph is just right. The fork angle is too steep for high speeds anyway. It'll get the shimmys I bet. I want to be able to ride it drunk. Going 70 mph on that thing would be dangerous enough sober.