Antique Photographs

One of the things I do is rescue very old photographs in poor condition from barn sales and estate sales and what not. 

I then scan them in and perform image restoration to make the photographs look almost new.

These images are clearly in the public domain, and both the photographer and subjects in them are long gone.

But the restoration is all my work.

So the question arises, would these require releases?

How would one secure releases from models or property owners who are long deceased, or indeed for property that may no longer exist?

What would be the proper procedure here.