you’d need a preop scan (call them anat_t1.nii and anat_t2.nii) and at least one postoperative MRI (postop_tra.nii) – call that one tra even if it’s sagittal. If you have multiple postop scans, you can call them postop_tra.nii, postop_cor.nii and postop_sag.nii
Now if you don’t have any baseline preop scan it’s hard to do it (given the lead artifacts will have a nonlinear impact on the normalization step). What you can do is to use the Linear Three-step protocol (but do make sure to read the associated schönecker paper so that you understand what really happens there).
If you want to try that, you’d make a duplicate of the single postop scan and call one copy postop_tra.nii, the other copy anat_t*.nii