Not all words have an equivalent kanji form, as is the case with いざ. I couldn't find any sources indicating a kanji form for this one, so it seems likely that it doesn't have one.
If you allow me to simplify the story a bit, words of Japanese origin (that is, not imported through Chinese writings like many jukugo) only really got kanji forms in 3 ways: there was a Chinese character with the same meaning as a word in the language word, the word went through ateji (that is, taking a character for its pronunciation only, which happened a lot but not always), or they invented a new character (these are called kokuji), so it's very much possible for Japanese words (especially of a more spoken than literary variety) not have a corresponding kanji form.
Even before hiragana was invented (and mind you, it was a very long time ago) there were ways of writing Japanese that used Chinese characters only for their pronunciations (like a more structured form of the modern ateji) and most of those forms have not stuck around. If you'd really like to know how it was written before hiragana was invented, you'd need to look into those forms of writing and diving down that deep can be pretty hard.
Does anybody know the original Kanji associated with this word in a historical context?
I understand this word shows up in many grammar points that use old Japanese grammar and words such as 〜ならいざ知らず」. I really want to know how this was written before Hiragana was invented.