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You could consider a wrecker such as Pinwall or MG Classics for a 2nd hand one.
Try pushing some corn oil up in there, that’s what I would do.
I have this switch. Not sure where I got it (perhaps Tom Formberg via Paul Linn?) but it has a Pinwall tag on it. Not even sure it's from a Guzzi. Does it look like yours? I robbed the signal button off of it for a customer's Special Sport, but it's otherwise intact. Pay shipping and it's yours.