While it’s clear how to use 2D Fourier transform, I’m not sure how Hilbert Spectrum can be generalized to a 2D case.
Any hint would be much appreciated!
One hint is to look into the literature. It seems that the 2D extension of the HT is not uniquely defined:
The Hilbert Transform (HT) and the analytic signal (AS) are widely used in their one-dimensional version for various applications. However, in the bi-dimensional (2D) case as occur for images, the definition of the 2D-HT is not unique and several approaches to it have been developed
The cited paper is freely available and comparatively practical and points out the difficulties. As far as I can see from a glance, equation (5) is what you need to apply after calculating the Fourier transform. The definitions for sgn are explicitly given under the Materials section.