This is the finned fixed half of the front pulley assembly, made of alloy and prone to failure on new Chinese made engines. Also known as "drive face", "fixed face", "fixed pulley", "belt pulley", and to some misinformed people, the 'flywheel'.
To suit Yamaha / Minarelli horizontal 50cc and 90cc engines with 16mm diameter cranks, as well as Chinese horizontal 2 strokes if your part looks like this one.
CPI 50cc from 2003 onward
Chinese scooters like Vmoto, Urban / Keeway 50cc R4 etc with 16mm crank