You will find several blueprint pieces for another device in the town hall- check out the filing cabinets down in the basement. From here, head upstairs and you will have to sneak past the guard- if he sees you you get kicked out. The two doors upstairs lead to a hallway that circles around the statue room. First however, go into the door to your right up here to enter the mayor's office. You'll find another blueprint piece. Move the filing cabinet aside and pull out the safe. Butt stomp it to get a hammer and chisel.
With these in hand head for the statue room. Stop at the top step here and use egg breath to make the sensor beams visible. You will need to weave through these to make your way to the statues to alter them. You should find another blueprint piece in here as well. Once this is done it is mission accomplished in here and you can head back out into WhoVille.