You are not going to he able to do that speed with this bike unless its just for a minute or so on level ground. 45-50 is fine all day with mine but you are looking at the wrong bike. I wouldn't try to do that with any air cooled gy6 or any of the smaller scoots you are just wringing it out at that point.
What you need is a Honda Helix with a Utopia adjustable backrest. http://www.utpr.com/
I Love my Rowdy to no end but it is more for city riding or surface street or where you can do 45-50 without getting in the way.