I have no clue about this machine, though in search of something else I did stumble upon this site, which may be useful...they have many brands listed including National. I have no affiliation/connection or interest in this group, but they seem to have a lot of manuals available for download....not sure if under $20.00 is reasonable, but personally I find instruction manuals extremely useful and would find a machine minus manual a bit more time consuming/frustrating. Others may have better ideas or places to try, but at least this may be a start.
http://www.tias.com/stores/relics/
Anyway, it looks a lovely machine and it would be great if you could restore it and get it going.