It would have been positive earth when new, and looking at the leads in the picture, the negative is routed through the armoured conduit to the front of the car , it is still probably positive to the frame . I would double check just to make sure there are no modifications outside the scope of the picture.
If its a dynamo it will work on either ground , ammeter will work back to front unless the terminals are reversed and as stated the coil low tension terminals will also need reversing . The dynamo may need " flashing " over by briefly closing the cut out contacts on the voltage regulator after reversing the battery terminals. If it has a radio that might be a problem ! Obviously if there are any electronic additions they would need to be looked at for polarity sensitivity .