How do I calculate the torque required to turn a turbo-fan shaft? Where can I find this information?

From the power and the rotation speed, maybe? Depending on what data you already have.

Or from the air's speed, pressure temperature (...and turbulence...) before and after the fan... And then you need the fan's efficiency as well, it's far from perfect.

Accepting big uncertainties, you can derive these parameters from the aircraft's speed and the fan's thrust and diameter.

Or from FEM computation at each blade.

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You didn't tell neither which shaft you consider. Turbo-fans generally have two shafts, sometimes three, and occasionally have a gear.

Also be careful that turbofans work at varying conditions: aircraft's speed, altitude...

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Anyway, replacing the gas turbine with an electric motor would be even easier there than at a turboprop.