Troubleshooting a prewar whistle should not be any different than servicing a postwar whistle. The relay should pick up when the whistle button on the transformer is pressed. Bypass the relay by connecting transformer power to the motor directly and it should turn. If it does not, remove the brushplate and brushes, and clean everything with naphtha. Replace worn brushes if necessary. Oil the lower armature bearing, replace the armature, brushes and brushplate and apply power to the motor again. It should run. Once it runs, hook it back up to the relay and test again. Here is the service manual pages on postwar whistles, which are similar to their prewar counterparts. The information is applicable to all whistles.
Larry