Fred is right! I have a 76 PUCH w/a 70cc kit and biturbo, the ped will do 45-50 mph on level ground and I weight 235#. A lighter weight person would equal more speed.The ped has held together but you can tell that you are pushing it to the limit. Now braking is a different story. You have to give yourself a lot more distance to stop. A panic stop means two things can happen and both are bad, either an injury (aka collision or bail) or a pants cleaning operation.
My Derbi Atlantis w/a high proformance Terrigas exhaust kit runs around 65-70 mph. However, the Derbi is a scooter and can take the speed a lot better than my MOPED. Braking is a lot better too, front & rear disc's. Also the is almost 25 yrs difference in technology here too. The technology gap between older peds and newer peds is similar, new peds are more efficient than the older ones (i.e., more powerful)
If you are trying for speed on a ped, remember you've got to stop. Also remember that there are physical limits to your machine, pushing beyond those with a regular street machine could be life-threatening or fatal. Rich