I don't think I can possibly believe this was 50cc. At those speeds the aerodynamics play a huge part. That little motor would need to put out 30-40hp unless he had some serious fairings of Bonneville lineage. He was only 16 years old - where would he get all this secret racing stuff? He bolted on something big.
My basis for this is my Kawasaki Ninja 250 I used to have. It had 37 crank HP and would go about 110mph in a full tuck. The frontal area was probably more than a little moped with a kid in superman position, but still - 37hp. My Ducati Monster I have today has about 70 crank HP and that only goes about 120 - probably due to the lack of fairings slowing it down at those speeds.