I've never had a problem going over 50 either, in fact I was one of the first (possibly the first, though unverified so I go with "one of the" to stay honest) to hit 50 with one of these engines almost half a decade ago. the best I've managed is 65 miles per hour, and my current race bike breaks 50 with a 24 inch wheel and 40 tooth sprocket, and likely could break my 65 mile per hour record by a long shot if I were willing to sacrifice acceleration.
I know from experience that these motors can make the power to hit the speeds some of these miami guys claim, I just doubt that they can achieve the required RPM without either blowing their motors first run or having some ridiculous compound gearing setup. I'm spinning way over 11000 RPM which is big numbers even for an engine not made in china and I'm replacing parts every other week.
maybe I'll host a southern racing event, I know I can dominate most of those guys claiming to do 70+ mph on a road course. just gotta find something that isn't an oval track or a drag strip in the south. harder than it sounds