I am surprised someone still uses that program especially because MNI tools can read NIFTI now. For the bias correction, we have a couple of very old tools that may or may not work for you. You'll have to try and see. 3dUniformize will probably need to be told which values to use for low and high ranges, or you can try the auto_clip option. 3dSpatNorm is the other program which is even older, and I know even less about. See the help for each program for more info.