Reversing the wires to the track and holding the whs/horn a second or two will turn on the bell instead if equipped with one.
A diode in place of or addition (external button) to the ZW's rectifying disc will likely be the trick. (external button box could give you both whistle/bell AND skip the voltage boost that postwar wants, but cause new locos to surge speed while being blown....like John says, lots of threads, easy project)
