Carl, it is is a standard extension spring with straight ends to hook on. You can find the attachment points with a small light and a dental mirror. The one on the door is more obvious and is a horizontal tab at the bottom with a hole in it for the spring to loop into. The end attached to the body goes into a vertical fixed tab like it in front of the lower hinge and is hard to see without a mirror. If you need dimensions of the spring, let me know. You should be able to find one close or easy to modify in a hardware store. The door will not have to be removed for installation
I may have a maroon Berlina door in my stuff, will look this weekend when I take my haul to the swap meet. I'm sure I have an extra of the fiddly little springs; it's actually a complex little sucker, though a normal spring might work.
Carl, try to repair the one on your car.. I have a had experiences with a Fulvia where a replacement door didn't fit up because of variations in the pre-robotic assembly methods. I also know of this problem in Giulietta Berlinas. You will have to take the door off either way so a photo of what you have would be helpful. Then it's those pesky rivets to deal with on the hinge.... it's always something.
I found my gas door. It's not perfect, but if it's better than what you have I'm happy to let you have it. I'll take a pic this weekend. A bit bent, and someone bent the spring attachment tang, though you could bend back.