Thankfully, due to the work of researchers and authors, much has been learned over recent decades, often in the architects own words, to determine true "design intent".
You have people like Geoff Shackelford and his book on George Thomas, Rick Wolffe, et.al. with Tillinghast, Brad Klein (and Ron Whitten, I believe) unearthing Ross, Doak with Mackenzie, Bahto coming with Macdonald, Raynor, Banks, and other fellows like Wayne Morrison putting together research (and forthcoming book(s) on Flynn. I'm sure I've missed some, as well, such as Travis, Thompson, and the future book on Maxwell.
Old drawings, aerials, pictures, etc., are all invaluable for sure. But, any course looking to do a true restoration and seeking to determine the real "design intent" would be well-advised to first seek to understand what the creator of their course believed philosophically and generally about design concepts, features, and the building of courses.
They will be far ahead of the game, I believe, if they start the process with careful study of these recently published books.