Is natural yoghurt & soured cream a good replacement for khatta dahi (Indian sour yoghurt/'curd')
There is quite a big difference in the process of making dahi vs yoghurt (aside from pasteurisation), as yogurt is made with bacteria (called yogurt cultures) and curd is made by curdling milk with an acidic agent (like lemon juice).

I would personally use full-fat greek yogurt and add a little fresh lemon juice to the yogurt for a quick-fix. You could, however, follow this recipe to make sour yogurt. I see the author also suggests just adding lemon juice to yogurt as a faster solution.

Let me know if this worked for your recipe!