My method:
This tool will convert 2.0 tracks to 5.1. And sometimes with 2 channel deleted scenes, it is actually a downmix from surround, so the tool is able to intelligently revert it back to surround:
From naturally extracted and expanded soundscapes to full cinematic big-stage enhancement, Halo Upmix delivers all the control you need to fine-tune your surround mix to perfection.
nugenaudio.com
Then I use something like freesound.org (free) or .zapsplat.com (paid) to add in ambience to the other channels. Like traffic noise, restaurant noise, house ambience, etc. It can be helpful to add in ambience to the center channel as well especially in extended scenes where you want to match the ambience of the previous scene.
As is often the case with deleted scenes, the audio might be rough/unfinished meaning you may need to add in basic foley sounds, use noise reduction, EQ the dialogue, and adjust the volume.
The halo upmix tool can also be used to quickly add in music tracks in 5.1 without having to make multiple tracks and an LFE track yourself.
EDIT:
@Hal9000 halo upmix may be interesting to you as well.