SPM uses the smoothness of the noise and the topology of the search region to determine corrected P-values.  Hence to duplicate the inferences outside of SPM you'd have to have the complete RESEL count vector available, as stored in SPM.xVol.R in SPM.mat

If approximate inference is sufficient, and you know at least the FHWM of the noise smoothness, the snippet below will give you some idea of the corrected P-value for a given FWHM, assuming a spherical search region of a given radius.  More accurate P-values will be had by reading the help on spm_resels and specifying a more accurate search region, and, most accurately, using the RESEL vector from SPM.xVol.R of the relevant SPM results.