We do not have genuine pork Italian sausages here in Israel. When a recipe like Ragu or Bolognese calls for Italian sausage, what meat could be used as a substitute for ground pork, and what seasonings should I add to match the seasonings that are commonly found in 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.
If you have not already looked, check the vegetarian section of where-ever you get groceries. At least here in the USA there are several varieties of Italian "sausage" that are entirely meat free and kosher. YMMV, but I find them to be an entirely satisfactory substitute.
Bar Akiva, you are very lucky as this is a really easy problem to solve: just don't use sausage in your ragu!
Traditional ragus don't have sausage at all. The usual recipes call for minced beef or minced calf meat as a primary ingredient; to it you can add a quantity of minced pork to add more flavour (by adding fat), balancing on your taste between 50% pork/50% other meat, to 100% other meat plus a bit of bacon, to no pork at all.
We even have duck ragu, rabbit ragu, wild boar ragu...really, don't let yourself be limited by pork.
Traditional Bolognese is also totally without pork.