We actually do have a SISU already. It used to have a lot of issues but Kovanaama has been fixing it up. he has gotten it to a point of needing only the interiors, and the different kinds of turrets and turret alignments. It has been moved down his list of work in favor of his awesome new Land Rover model, that could benefit the British in most of their maps.
Interior reference pictures of the SISU has proven difficult to find though.
If Kovanaama still has a lot to do when you are finished with the Benelli, I am sure he will let you take a crack at finishing the model.
Of course, this is a very troubled model, ever since its creation. If you wish to create one from scratch, I will not stop you.