If you need a full carb gasket & oring set these should be available from a Dellorto agent.
The part number was DE52511.
There are a number of trumpets listed for VHB carbs.
DE4224 (most likely the standard fitment,)
The DE1439 and DE3100.
Again find a Dellorto agent and see what they have or search the internet.
On the internet you should be able to find Dellorto Carb manuals and parts lists for VHB's.
A better air filter option may be to get a modern replacement element
The Sei air filter element was also used on Ducati 750GT and I understand some Italian cars (Fiat 850 etc.)
Other forum members may be able to give you K&N or Fram part numbers