Unfortunately, no "general", simple, automatic process is likely to succeed at this task. See "How does LaTeX relate to Plain TeX" for further details.
Translating a document designed to work with LaTeX into one designed to work with Plain TeX is likely to amount to carefully including (or otherwise re-implementing) all those parts of LaTeX, beyond the provisions of Plain TeX, which the document uses.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=LaTeXtoPlain