There are solvents that dissolve any kind of old paint, possibly without attacking the surface of the hinge. Read the fine print.
And I guess a torch may help.
"Knife hinges are a variety of concealed hinges that require a slot or saw cut into the door to house the "knife" (some types that mount on the top or bottom of the door don't require these cuts). The hinge mounts to the back of the door and to the edge of the face frame."
Depending on what woodworking tools you have, you may be able to adapt other hinges with similar geometries to your application.
These "three dimensional tasks" are hard to document on two dimensional paper so ask lots of questions and make lots of drawings and photos, from all three angles and with hinges both opened and closed.
Even with all this, it may be 50-50 you don't need to return any parts.