Set a large pot of salted water to boil. In a large skillet, cook bacon over medium heat, stirring occasionally, until crisp, 8 to 10 minutes. With a slotted spoon, transfer bacon to paper towels to drain. Pour off all but 2 tablespoons fat from skillet. Add leeks, season with salt and pepper, and cook, stirring often, until leeks are golden brown, about 10 minutes.
Add pasta to pot and cook according to package instructions. In a large bowl, whisk together eggs, Parmesan, and lemon zest and juice. Whisk 1/4 cup pasta water into egg mixture.
Drain pasta and immediately add to egg mixture, along with bacon, leeks, and parsley. Season with salt and pepper and stir to combine. Sprinkle with more cheese if desired and serve immediately.