It depends of your problem. Astronomer and atmospheric scientist use phase screen and Fourier transform. You might need Monte-Carlo raytracing in your case.
It seems that you have two questions. 1. How to simulate the SHG of a given pattern, and 2. the point spread function.
For the question 1, you may use Green function for far field SHG calculation, in which the governing equation you can find in most nonlinear optics books.
For the question 2, what do you means of "point spread function (PSF)"? Is it PSF of your microscope? or the PSF of the SHG signal coming from a point of the tissue?