The power output of a (practical) Stirling engine is dependent on the physical size of the engine, specific design details and as far as I know, has no calculable relationship to the temperatures of the heat source and sink. Those temperatures relate to the maximum efficiencey possible for a given range.
The greater the difference in operating temperatures the greater the potential power output. The 'source' temperature is limited to the available heating system and the stability of the hot end materials of construction. The 'sink' temperature being limited by practical considerations to ambient air temp. or cooling water source temp.