If you take the handle off, put a big chunk of wood back in its place and shape it correctly in the fades you can fix it. I have filled over cut sight windows as well but I don't think this bow needs it.
That crack is going to take a ton of superglue as well after you cut down as far as you dare toward the back.
Here is how to fill a deep cut out and make it look like it was the plan, you don't try to fil it all. Sorry for the bad picture, it was a thumbnail in my bow making album that I resized to make bigger.