Alliance windows, plastic balance coil cover piece has disintegrated

The white plastic cover for the flat coil-based balance spring has weakened in the sun, causing it to crack, making it difficult to open and close the double hung window. It appears the shoes and the coils themselves are intact, though perhaps one of the coils is a little bent. The locks on the windows say the brand name "Alliance," though I do not see any markings elsewhere on the windows with the exception of the three coil springs which I believe reads: P 2208 35". Thanks for the help!