If you are using a desktop computer it’s an easy fix. 1. Turn mouse over. 2. Carefully remove ball. 3. Scrape off under seal from mouse until you get back to good plastic 4. Spray with rustoleum 5. Good for another forty years or until it gets wet.
You might need to change the little rotary mechanical switches on your 1990's mouse too. Just as well they use lasers or LEDs with a little camera watching the texture of your desk or leg slide past the camera in modern computer mice.