Here is how I might do it, keeping in mind I'm not a by the book guy when it comes to following every little rule in 3e.
1. Take the familiar as it was at the time of its masters death. Adjust the mental stats as needed and double hits HP, we'll say it absorbed some its masters power and gained the ability to speak (sort of like it became awakened).
2. Then either give it how ever many wizard or sorcerer levels you need it to have to work for your game. Wizard if the idea was that it absorbed some its masters knowledge or learned how from watching its master in life. Sorcerer if it has simply absorbed so much magical energy over the years that it has become magical itself. You'll probably need to have the still magic feat (and so it'll probably have to cast spells almost as if it's two levels lower so keep that in mind when assigning its level) and the eschew materials feat.
3. Terrify party with small woodland animal.