Sorry to hear about the issue you are having. I had mine yellowed badly as well and decided to go for a totally new and modern looking front-end instead of just swapping the melted part.... which is not available btw
This:
Also offered in the color of your choice! High Performance Bi-Xenon Projector for Porsche headlight (RETROFIT KIT).
Over 300 bucks later, however some of the best headlight beam you'll ever had on any of your cars. Makes the litronic look like a rip-off once installed
Read at:
Porsche Headlight Retrofit [from CAD, prototyping to finish)