Have a look on www.benelliparts.de.
Click 'Benelli Teileshop'.
Find the appropriate model.
Select 'Motorgehause Zundung' from the drop down list.
You can then look at the part diagram and click on the numbers to see what a part is called and its price.
This is a 250 2C engine (not sure what 250 you have?). Part 31 (in two places) is indeed a gasket.