National used "top clamp" feet, so they will require a little bit of searching to find them. Aftermarket "Griest" brand is likely your best bet.
You are probably going to need a feed dog cover to do free motion. I think a generic cover will work.
This site covers a slightly newer model, the "Streamliner", but the two are similar so their technical information might be useful:
http://www.doubleveil.net/zssmp/streamliner.htm
I have found little service info on Nationals due to them being a multi-brand department store machine.