Andrew,
This is a really tough question with no simple answer. Golf courses (including the greens) are constantly changing and evolving from the “original” design. There is really no such thing as an original green that has not been touched in away way (outside of a green on opening day when it was first built). After that, most greens get mowed, scalped, settle, get top dressed, aerified, deep tined, verticut, fertilized, regrassed,…, the list goes on. Hopefully they don't get rebuilt and/or moved but that of course happens as well. Pinehurst #2 greens are a good example. They are about as far from the “originals” as you can get.
Restoration to me is along the lines of what Tom said - restoring original “intent”. Even that is subjective but original plans/drawings, old photos, old aerials, written descriptions,… all help the cause if they can be uncovered. Soil probing can be very helpful as well. We use that a lot for example in bunkers to determine the original bunker shapes and floors. Sometimes you can find bunkers that have literally three feet or more of sand in them. Restoration of the famous Taft Bunker at Myopia Hunt Club is one example. When “restoring” that bunker, they kept digging sand out of that bunker until they finally gave up (that is how deep it was).
Greens tend to shrink (as do most things except trees on a golf course). Restoring a green's size out to the edges of their fill pads is one of the easiest and most helpful things one can do to “restore” original design intent. This helps reconnect the greens to their surrounds/hazards, restores some of the best hole locations and makes the whole green complex more fun and interesting for all skill levels of golfers. It takes some time and patience to get it right (figure a good year or two in most cases) but the enhancement to most greens is amazing.
I could go on but hope this helps.
Mark