Piaggio SI V-belt vibrates heavily?

Hello

I have the following problem:

On my Piaggio SI the belt swings so much that it tears it.

Full unscrewing is not possible.

A Polini SE housing with Polini 72ccm, 16/16 Dellorto is installed

new longer gearbox (CIF)

Polini Vario

Gianelli Fire exhaust

Replaced later due to vibrations:

new Malossi Fly Clutch converter

New Mazzucchelli Racing crankshaft including bearings etc.

Original flywheel (previously lighter, keyway was a little worn)

(Malossi X-Belt belt)

Tried different tensions.

I'm at my wit's end. What could this be????