Can I substitute regular milk (or something else I might actually have in my house) for the milk powder, and if so- how much? Thank you!

I have found that you can use non dried milk instead of part of the water. In this case about 3 Tbsp. Milk (dried or not) does have a chemical reaction to the other ingredients. I wouldn't leave it out completely.

In the comments of the recipe someone weighed in saying: "I have made this bread with and without milk powder. In my experience, milk powder gives a little bit of a "milky" taste but even without it, it does not change the texture or taste of the bread that much (it is ONLY 3 tablespoons...) Don't stress yourself out not finding milk powder. Just make the bread without and perhaps add a tiny bit more of flour if you think the dough is a bit on the wet side. Good luck and let us know how it goes!"