Indeed, the 50cc Rieju's use Minarelli AM 6 engine.
I still have my old Aprilia RS 50 from 1998. This uses a minarelli AM 5 engine,but in some other countries the AM6.
With 26mm carb, good exhaust and 76cc cylinder, it pushes around 120-125kph. Fun stuff!
And speaking of 175cc;
A good friend of mine still has his Yamaha DT175 from the late seventies, or was it early eighties..

It's a nice and very fun bike!