Overhaul the whistle motor to start with. Clean off all old oil and dirt with naptha (lighter fluid), especially the commutator and brushes, and lightly oil both the top and bottom armature bearings. Look at the contact points of the relay, and if they are pitted, sand them with very fine crocus cloth, and then clean the contacts of the residue.
You may also need to run the engine and tender with a longer train, so that more voltage is supplied to the track to help spin the whistle motor properly.
Larry