Put the potatoes in a microwave-safe bowl, add 1/2 cup water and season with salt. Cover with plastic wrap and microwave 5 minutes.
Meanwhile, heat the vegetable oil in a medium skillet over high heat, then add the squash and cook until browned but still crisp, 3 to 4 minutes. Season with salt and pepper and transfer to a bowl. Add the sausage to the skillet and cook, breaking it up with a spoon, until it begins to brown, about 5 minutes. Add the potatoes and cooking water and cook, tossing, until the potatoes are browned and the sausage is cooked through, 5 to 6 more minutes. Season with salt.
Combine the tomatoes, scallion, jalapeno and cilantro in a bowl and season with salt. Warm the tortillas in a dry skillet or wrap in a damp towel and warm in the microwave, about 1 minute. Fill with the squash and chorizo mixture and sprinkle with the cheese and salsa. Top with sour cream and/or guacamole, if desired.