The amount of power your traditional water mill extracts would be exceeded by solar panels on its roof, which would be cheaper than the building under them, not even counting the cost of the water wheel and weir. The mill might last centuries, but only with continuous maintenance.

A weir way uphill with a penstock and Pelton wheel could do better than the old mill, but both depend on landform features with limited distribution. If you needed twice the power, you would be stuck. But you can put out more panels.