Hello! Super cute idea! I think you can carve the cat from cake or use half of the Wilton's sports ball pan and cover in fondant. Use fondant/gum paste for the ears. You can make the hat from 4 or 6 inch round cakes and use just a cardboard cake circle to support it and dowel the cat's head for support. It should be ok to use a Wilton white plastic dowel through the center of the hat at an angle to dowel it. The brim of the hat can just be a dried gum paste circle. HTH!
If this cake is for a customer you'll probably also want to obtain permission from Dr. Seuss to use a likeness of Cat in the Hat in order to avoid copyright infringement.