It is often the case that one or more parts turn up missing on a vintage miter box. Replacing them often means salvaging components from another less fortunate box or making them yourself.
To the extent that I have been able to measure various parts directly the measurements have been included in the slideshows for the various models. In addition, I have posted links to some of them at the bottom of this page.
Depth stops, length stops and stock guides are the easiest for the home enthusiast to fabricate.
The parts that are probably the most difficult to obtain and which are the least transferable from one model of miter box to another, the saw guides, can be successfully made of wood if you do not have cost-effective access to a machine shop. The same may also be said for the old style of cast iron bracket feet.
In older (pre-1930) miter boxes, the posts in which the saw guides slide vertically up and down are often found with brazed or welded repairs at the point where the post attaches to the swing arm. As long as the saw guide is held steadily yet remains free to move vertically then the miter box should work just fine. (This might be of particular interest to the unfortunate auction winners whose prizes got broken during shipment.)
Backsaws are commonly available on auction sites and through antique dealers. You just have to be careful that the saw you buy fits the guides in your box. How do you know if the saw will fit? You measure the length of the slit in the guide, with allowance for the spine (and bearings, if present). The teeth of the saw must land anywhere within the extra-wide portion at the bottom of the slit, as shown in the following photos. The length of the extra-wide portion (about an inch) allows for the saw to be sharpened and retoothed over its lifetime and thus lose some depth of cut, yet continue to fit your miter box.
Two contemporary manufacturers of high quality new backsaws are: Mike Wenzloff and Mark Harel. Their products are excellent.
Photo galleries of frequently missing parts, with measurements for making your own, appear below. Similar contributions from readers of this web site will be gratefully accepted.
74C circa 1950 through 1980, numerous parts (scroll to bottom of the page).
74A and Craftsman 3650, circa 1938-1940’s, cylindrical stops, courtesy of Rick Stillmaker.
Depth of internal hole is 7/8 
3/8-16 X 1-1/2 
9/16 outside diameter 
1-5/8 
3/8-16 
74A and Craftsman 3650, circa 1938-1940’s, length stops, courtesy of David Toone.
A Langdon Mitre Box Reference 