Yeah.. those speed claims you've been seeing are probably all for European spec bikes.. and Jan was right, their bikes are way cooler stuff than what we get (you can thank government controls for that... don't vote for GORE)... the ones that do those speeds probably all have gearbox's.
We are stuck with the slower, speed limited stuff with belt drives and restricted HP, and they are not really made to go fast.. so if you do make them go fast, you are overstressing them.. and quite often overrevving them...want to see my PA-50 that I modified to go 35 mph? (from the stock 21mph).. the crankshaft end broke right off from revving too high.. I was getting ready to shim the pulley to gear it higher and lower the revs.... but... snap!... and as Ree said.. the brakes are pretty wimpy too.
Or buy an old small motorcycle if you want to do 70 with reliability and safety.