Make a cardboard prfile of what you have or what you will have when the old is removed. Take it to the boneyard and choose a roof that best suits. If you do the cutting out or whoever does take plenty and trim when you get it back home.
Remember also to consider the length. Want the ribs to end with hanging over the end of your roof.
I think if you do enough reserch here and on other rod sites you will find some pictures. Maybe someone here can post.