Replacing a piezoelectric speaker with an electromagnetic one
By simply attaching a low resistance EM speaker you will not be able to keep Q2 working, see the top part of my solution posted in the image. However it is possible to accomplish with another transistor. Create a two stage amplifier where the output stage is an emitter follower. This will buffer the volume amplifier created with Q2, allowing you to drive the low resistance speaker. You will not have great battery life and your volume will tend to be less than what the piezoelectric speaker produced but it will certainly work. If you find you need more gain increase the 1K ohm resistors used to bias the base of Q3. Make sure the base equivalent resistance allows for enough dc current to keep Q3 on.

PS: This solution assumes you have a greater than or equal to 32 ohm speaker.

Let me know if you need more explanation or help.

-Ryan