How to increase amplitude in given oscillator circuit?
Have a look at this video by Big Clive where he reverse engineers a water atomizer: https://www.youtube.com/watch?v=DabnOLtLFnc 

At 13 minutes into the video he shows the schematic:



Note how it uses a transformer to make a high voltage. This is needed because the transducer has a very high impedance meaning it requires a high voltage at a low current in order for any power to be transferred to it.