You don’t need or want a Fizban stat block. Normally in these articles I take a famous D&D character that doesn’t have an officially published 5e stat block and I put one together for you. You can check in on Themberchaud, Mordenkainen, Szass Tam and the rest here. I was going to give Fizban the same treatment. The thing is, Fizban’s not that type of character.
Fizban was introduced to fifth edition with Fizban’s Treasury of Dragons. Wizards of the Coast published an article on Fizban explaining his background. He was originally an NPC in an actual game of D&D played by the authors of the Dragonlance novels. You can get more detail on that from the WotC article I linked above. Here I’m just going to talk about the type of NPC that Fizban is.
Why Don’t I Need a Fizban Stat Block
You don’t need it because Fizban doesn’t engage in the game in a way that requires it. Indeed, a Fizban-type character would be worse for it, as it might encourage you to actually use him that way. A “Fizban” gently move the players along when needed. He provides bits of exposition as needed, and fun roleplay when the time is right. Fizban is the type of character who leaves the game to the players.
Avoid temptation. This is a “less is more” situation. If you know what spells he casts, or skills he has, you might step in and solve problems for the players. The players might even grow dependent on your NPCs abilities. You really don’t want that. DMs set the obstacles, players overcome them. Maybe, every once in a while, a bumbling old wizard like Fizban wanders through the scene and trips over the McGuffin that the players haven’t been able to find for so long that the game has stalled, but that’s the most a “Fizban” should do.