Unless you have access to some pretty good filtering software, turning the mono mix into stereo would be very hard if not impossible. You have to have a way to filter each individual sound and musical instrument and then rechannel it form the single mono channel to match the left/right placement in the stereo mix. I’ve thought about this and the easiest way I’ve settled on is to take the best match Dolby Stereo mix and split out the channels and replace the center channel with a filtered version the mono mix where it differs and then clean the other tracks to make sure no cross over sounds from the stereo mix interfere with the new mono mix sourced center channel. That may prove nearly impossible, but it sounds easier to me than filtering the mono mix.