Decoding Korean text files from the 90s

All of those octal codes that emacs revealed are less than 254 (or \376 in octal), so it looks like one of those old pre-Unicode fonts that just used it's own mapping in the ASCII range. If this is right, you'll just have to try to figure out what font it was intended for, find it and perhaps do the conversion yourself.

It's a pain. Many years ago I did something similar for some popular pre-Unicode Greek fonts: http://litot.es/unicode-converter/ (the code: https://github.com/seanredmond/Encoding-Converter)