you could maybe stick it back together our if your still have the shape you could draw around it but i thin k 3D printing would be okay
sorry if i wasn't any help
I am not sure which piece you mean by "middle head piece" but it if is the one which accepts the head servo then that is the piece that I broke over the course of several iterations of taking apart and putting back together the head for various reasons. It broke in that notched area where the servo wire passes through. In my case I found that the servo mount screws held that part together just fine, although I did think about using a piece of the scrap board material as a splice which is something you could try as well. And I just saw that Rz has sent you the file to 3D print the part - sounds like you have a 3D printer and that seems like a good and fun fix.
The format is not supported on the forum. I have sent you the STL through email.