A few comments:
1. The areas on that print that are missing were probably ripped off by someone trying to separate the print from the glass. Do not add to the problem. Unless you are working with a museum-level conservator who knows how to chemically separate the two (without destroying the materials), you are better leaving well enough alone and just scan or photograph through the glass.
2. Everyone who mentioned the importance of scanning at higher resolution was absolutely correct.
3. However, scan at exactly the native optical resolution of the scanner (look this up). Don't scan below this value (which most people wouldn't do), but, just as importantly, don't be tempted to scan above this value. The ultra-high resolutions often quoted by the manufacturers is just the result of the scanner up-rez'ing the data for you -- essentially a sales trick. You are better doing this operation yourself in PS, should it be needed (which is very unlikely). Even for my scanner, a V750, a relatively high end flatbed, the native optical resolution is only around 2500 ppi. Most others will likely be less.
4. Higher resolution in the digitization step will be
utterly useless unless you get the image in focus. A few pro flatbeds allow software adjustment of the focus plane. Most don't. Either take your image to someone who has such a scanner, or take it to someone who will digitize your original by rephotographing it instead of scanning it. They will be able to get it in focus. Re-photographing a print behind glass is an art unto itself. Don't think about doing this yourself unless you are an expert and have suitable equipment (eg, at least 3 or 4 polarized light sources, a crossed polarizer on your camera, stands for careful positioning of your lights and camera, a flat field lens, etc.).
5. You are going to want to extract every bit of local contrast possible from this print. Scan (or re-photograph) at a bit depth of 16 bits per channel. You don't want the output of the scanning software to be an 8 bpc JPG. If someone offers to scan it for you but can only produce 8 bpc output, go somewhere else.
6. Most pro retouchers will disagree with the suggestion to scan in B&W. The reason is that when a print is discolored in areas or has some areas that are adhered to glass and other areas un-adhered, often there are small color differences that can help enormously in distinguishing these areas for separate post processing. (see the section in the book mentioned below for a full discussion of this).
6. Gather together any other prints or slides of family members from that era. You may be able to use bits and pieces of individuals in those to reconstruct the missing pieces of this print.
7. Buy, read, memorize and practice everything in this book before you work in earnest on your own important print:
http://www.amazon.com/Digital-Restoration-Start-Finish-photographs/dp/0240812085
...almost everything I said above is in that book. I own several books on this topic. Newer books are available (eg, the ones by Katrin Eisemann), but IMHO, C'Tien's is by far the best nuts-and-bolts, real-world guide.
8. I have to agree with Fatboy's suggestion to consider hiring an expert to do the work for you. IMHO, even if you are already an expert in normal photographic uses of Photoshop and constructing images "from nothing", etc., it still probably takes most people weeks of practice on dozens of mangled prints before they become really good at restoration of prints like this.
HTH,
Tom M
maybe because I don't know what to do with it. 
