There is a guy with a shop in Vancouver, BC, CAN, that should have that drop target in NOS. Not cheap, but if you only need one or two ... ?

I am not associated with the shop, but bought one for my Grand Prix recently to replace a target that shattered.
https://www.flippers.com/catalog/product_info.php/drop-target-bullseye-p-5544

If that's too rich for your budget, I have one that I could take measurements for you. What exactly do you need to know?

Mylar should protect the touched up paint on the bulls eye, but it probably won't prevent breakage if the target gets hit hard enough. You'd also likely need to cut out matching holes where the rivets are, or the mylar will not lay down flat and will likely look odd.