Simulating a magnetic field in Spice
Model the change in field as a single ac voltage source. Add to this a resistance and inductance (pickup winding resistance and leakage inductance). For even better accuracy add some capacitance to model a bulk interwinding capacitance:

Lleakage 1 N1 &lt;value&gt;

Vac 2 N2 &lt;func_of_time&gt;

Rwinding 2 1  &lt;value&gt;

Cparasitic N1 N2 &lt;value&gt;