[Reece]

Do you still have the old speaker? You're going to need the output transformer. Also the old speaker had a field coil which serves as a choke for the power supply. Your options are to have the old speaker repaired (reconed) if it needs it and if you still have it, or to use a new speaker with the old transformer or an equivalent one, and provide a choke or appropriate resistor and larger electrolytic filter capacitors in the power supply. If there's room in the cabinet and you have the old speaker then the field coil can stay connected and the old speaker screwed down somewhere while the new one goes into position on the grile. Need more information!