LED might be hard to see, those are typically IR, only a tiny red spot can be seen in a dark room and if looking straight. Replacing LED might not be as straight forward as you think. That scale might have more than one of them and any misalignment between old and new LED or difference in intensity would cause scale not to work or work with degraded accuracy. Calibration and/or adjustments will be necessary which requires specialized tools and experience.
Considering the age of that scale, very likely other parts are worn already out or very close to failure point. Replacing the entire scale would probably make more sense.