Which crankshaft do I need?

I’m changing a Vespa 125 Primavera ET3 125cc cylinder with the Polini 50cc kit and I need a new crankshaft too.

Which crankshaft (with the correct piston pin Bearing) do I need to choose which will fit in the Vespa 125 Primavera motor and the Polini piston? Whithout having to modify the crankshaft. Thanks for helping me out!