What is a non-pork substitute for Italian sausage?
By "Italian Sausage" I think you mean the seasoned pork sausage available in many supermarkets throughout the US. 

I've found that a 30-70 mix of beef and turkey/chicken works reasonably well as a substitute when pork is not available. Beef is too strong a flavor and turkey too weak in its own. Flavor-wise most italian sausage has red wine, fennel, and oregano.