i have a DataObject which is managed by ModelAdmin via the cms, so far so good. But all objects that are related to this with a $has_one relation are only shown as DropDownField with the id. How can i customize this to change the DrowDown to other fields from the managed object?
I'm not sure what you mean by "How can i customize this to change the DrowDown to other fields from the managed object". Since this is a relationship within the datamodel you are restricted to what can be used... the option I think that might help is the ItemPicker...
Where the system is trying to scaffold the drop down it looks for a "Name" field. If you add that then it will look a bit better.
Alternatively - you could redraw the drop down with the right "Name" (within getCMSFields)
$dos = DataObject::get($strDataObjectName);
$map = $dos ? $dos->toDropdownMap('ID','Name') : array(); //change Name in this line