Filtering the fundamental component from a proximity probe signal

Hi, I have a proximity probe signal with sampl. frequency 5000 Hz. I have found the fundamental component using FFT, but I am not sure how to get this frequency back to the time domain using the inverse transform.

The idea is to get the 1x filtered signals from X and Y prox.probes and do an orbit plot.

I am not putting any code since finding the fundamental component using FFT is trivial and has been answered before in the community.

This is a industry project, so any help would be valuable.