Please note that inductor coupling in spice will not actually represent a real world transformer with any degree of accuracy. There are some transformer models on the Yahoo Groups LTSpice group, but not only are these models overly complicated for what you need they require a lot more information about the materials in the transformer than you probably have. There's really no point in simulating this in the first place as already mentioned. Simply use a dummy load resistor, tweak that resistors value until you get peak power output from your circuit, using that resistors value, and determining what the output impedance of your transducer is will give you the impedance you need to calculate the proper turns ratio and wire thickness of the primary and secondary of your transformer.
If all you want to do is simulate the amp output, all you need to do is use a dummy resistor with a value that matches the impedance of your piezo transducer at the frequency you're producing. If you don't know what that impedance value is you have nothing to simulate.