Hello,
i’ll try to explain in english, i’m from italy.
You could try to cut a piece from a can of beer or cocacola and insert it in the exhaust collectior.
Tin is a very soft metal and will squish as you tight the bolt. You could add a bit of paste if needed.
I did this on my original piaggio si and it worked flawlessy.
As for the exhatust type, Giannelli is a bit rumorous, in my experience if you switch it with a Sito Plus it will be much more silent.