Explain the current situation better. Are the "radiators made of natural stone" night storage heaters or are they heated with water that is centrally heated electrically? In the second case, you only need to install a heating system where the large boiler is currently located. In the other case, pipes have to be laid throughout the house. Having enough electricity available from photovoltaics to cover the heating demand is unrealistic. It is not enough even in the evening during transition periods and certainly not in winter. The open fireplace is presumably no longer allowed after a change of ownership.