The easiest swap is chickpea tofu or pumpkin seed tofu. You could probably also use JUST Egg. A can of chickpeas, or one-inch thick rounds of butternut squash or sweet potato also work. If you do use squash or potato though, allow them extra time to cook. Covering the pan with a lid to let them steam a bit at first might be useful, then remove the lid once they are tender to allow the sauce to reduce and thicken up.