Old damage or was the lens new when you installed the led?I bought a festoon style lamp from superbrightleds.com. They have a lot of variety - just match your lamp's dimensions. Mine looks like this:
Only one problem: I believe the light is deteriorating the plastic on my dome cover. There's a dime-sized crazed spot right at the hot spot. I know it's not heat, because the lamp is cool enough to touch when on. UV rays maybe? They can destroy plastic.
It was the original lens, but I swear the crazing wasn't there before the LED upgrade. However, I see your point. 28 year old plastic next to a hot (incandescent) lamp for that long will behave differently.Old damage or was the lens new when you installed the led?