Afraid I can't give specifics, but my neighbor did a beautiful and seamless job using the type of product one buys to make molds of the hands of children. You have the child put his hand in this "rubber-plastic" product (available at Michaels) and then pour plaster of paris into the resulting mold. Well, you take the "rubber plastic" product and press to over an intact part of the frame that looks like the missing piece of frame. When you pick up the product, you will have a mold of the detail of the frame. Pour in plaster of paris and let dry completely. Peel mold off and you will have and exact replica. Just glue piece into empty space. I really, really, looked at my neighbor's frame and could not see where the old decoration left off and her repair began. Good luck!
Looks like I will be making a trip to Michaels!
You can do the same thing using silicone or even hotmelt glue gun. give the part you want to copy a spray of Pam before covering it with water you use so that it releases easily. Let it dry and pull off carefully. There was a good posting on this today on the email that I received, here is the link to it.