I don't usually keep buttermilk at home but make it with milk and lemon juice when I need it - will this concoction work in this recipe?

Milk + Lemon Juice
While the quantity of acid could be fine-tuned, the consistency of milk-based substitutions will be unavoidably thin. Compared to cultured buttermilk, plain milk is watery, making the dough so heavy and wet that it oozes into a puddle, turning the biscuits or muffins flat and dense.