It looks like the main dictionary uses the ASPELL system. en_US.dic is clear text but has modifiers attached that give variants (like adding 'ing' or 'ion' to the root word). The modifiers are defined in en_US.aff, which is also clear text. There is a version of ASPELL for Windows. ASPELL is well documented.
I did not research it but there is supposed to be an easy way to convert an ASPELL dictionary file to an LSPELL file, which is a complete word list with all the variants included.