I was under the impression that Scientific American had started selling scans of their oldest issues to DVD, but I seem to be mistaken. Maybe I'm just thinking of the "Amateur Scientist" CD from a few years back.
Anyway, I did some poking around that ended in complete failure. However, you might want to try getting in touch with Rick Prelinger's team. I think of the Prelinger Archives as this site for the rest of the universe, except that they're primarily a library.
http://www.prelingerlibrary.org/They're associated with the Internet Archive (archive.org, if you haven't seen it--there's a devoted Prelinger section), so they obviously have intentions of scanning things, and I do see Mechanix Illustrated on their list. There are obvious copyright issues to clear before scanning them, of course, but they might be good people to talk to, and while I've never personally been in contact with him (something that I really should rectify), I hear Rick is a great guy.
Alternatively, if you live near San Francisco, they ARE a real library with (as I read the page) visiting hours and an offer to use their scanner.