Using the Pythagorean theorem, we can find the length of the diagonal fence:
diagonal²= length² + width²
diagonal²= 120² + 75²
diagonal² = 14400 + 5625
diagonal²= 20025
diagonal = √20025
diagonal =141.5 feet
Therefore, approximately 141.5 feet of fencing are needed to make a diagonal fence for the lot. Rounded to the nearest foot, the answer is 142 feet.