You don't need to add a volume control, it already has one. What do you think the 10K pot is for? It should have an "audio taper" (logarithmic).
Lots of luck with this extremely, extremely, extremely, extremely, simple super-regen FM radio!
Forget stereo for it.
If your area has lots of stations then tuning it will be a pain.
It will pickup AM too. Tune it a little above the FM band and it will pickup airplanes and air traffic controllers, if it's near an airport. Its radiation might even interfere with them.