At the time, “to speak French” was translated as “parler le François” that became “parler le Français”.
It seems that what was written (and pronounced) “oi” became “ai”, as according to this book on Middle Ages dialects: “je fois” stand for “je fais”.
The other variations are quite similar to contemporary French though. From what I understand the pronunciations would not differ.
According to this Histoire the spelling followed the pronunciation until about the XIII century.
So (I don’t but) do you know how "faire" was spelled in the X through XII centuries? That might give a clue about pronunciation.
In Québécois (which is pronounced like Kay-Bek-Way or Kay-Bay-Kway, and whose pronunciation I assume is closer to an older French because e.g. it retains phonemic distinctions), "faire" is pronounced with a bit of a dipthong,
In Quebec French, long vowels are generally diphthongized in informal speech when stressed.
[aɛ̯] as in père "father"
In other words I’d guess there’s some distinction between the "a" and the "i" in "faire": perhaps like "fa-ïre" or perhaps like "fa-erre".