looks like bad case of Fowl Pox. I would separate those showing symptoms to their own area and cover their sides of the pen with sheets to help slow the spread. Apply Iodine to the scabs. Watch those that may develop into wet Pox, that is more severe and causes respiratory illness. If that happens start them on some tylan 50. Sometimes some childrens cold & flu med helps( i use Wal brand). Other than vaccination those not yet affected, treat and give supportive care until it runs it's course.