For the retrofit of old Märklin all current motors you need a permanent magnet. It replaces the present field winding, and in combination with a LokSound or LokPilot decoder, helps to make your loco run astonishingly smooth.
You find the armature code number on a spare-parts sheet, which you can download from www.maerklin.de.
Permanent Magnet as No. 235690, for Anker 231440, D=19.1mm, for Engine Plate 231350