When I was a kid my moped did run 90kmh with 50cc modified cyl, modified piston, dropped cyl-head, 5-speed trans and modified carb.
Cop who measured the speed with another one in 1000cc BMW said the fuel-mix wasn't at its best so engine did run a bit rough at higher rpm. Carb had a adjustable mainjet the cop didn't know how to use...
Friend had a Suzuki moped with 80cc motocross engine; topspeed 120kmh.
Another one had a old Honda Monkey with 125cc Honda
Twin; he had to tilt engine forward so it could fit between frame and ground. It was quite fast too
Third one was mounting an engine from old Ancillotti endurobike to his trial moped when his dad said "Son, guess we should buy you a motorbike..."
Oh well, party time...