A 212cc preddy should be able to hit 60mph, and a 4.5 briggs should atleast approach.
I've gone atleast 55 or so on my lawnmower bike, according to an online go cart speed calculator. That's with a 140cc 2 stroke mower engine (stock 4hp but it has some work for sure). But cruising at that speed? Daily? You will die for sure, unless you use a proper motorcycle frame. My lawnmower bike is straight as an arrow at high speed, but I've only taken it that fast once, maybe twice. It's a very sketchy bike, so I rarely ever ride it anyways.
Converting a vert. Shaft to a horizontal won't be perfect and you'll probably still get some oil starvation. Either way, maintaining 60mph daily with a cheapo briggs will kill the engine within a few weeks probably, unless you gear it so it isn't over-revving so much (anything over 3600 will eventually cause the big end bearing to heat seize, and throw the rod)
And what about flywheel explosions?? Happens all the time with the racing mower guys; the cheap cast iron flywheels could fly apart and kill you. But since you have a mower engine it'll be an aluminum flywheel, which causes problems of its own. My 2 stroke mower bike has an alum. Flywheel and it runs VERY rough. I once tried
Starting a 4 stroke briggs 6.5hp without the blade acting as a flywheel and it kicked back almost every pull. It would tear the handle out of my hand and smash it into my ribs painfully hard. Even my lawnmower bike kicks back pretty bad at times; I need to slowly pull it to TDC, then pull it to prevent it from kicking back. (Old powersport trick)
If you search up "lawn mower blow up" or something like that, there's one video where the guy actually removes the governor so it revs wide open. It threw a rod in a few minutes. So unless our stretch of 65mph highway is 1/2 a km long then you're asking for trouble
Also, a stock briggs with no governor will only rev to 5500rpm or so, because the valves start floating.