hello, had exactly the same problem with my classic. i resorted to taking the head liner out via the rear tailgate, removing the surface material and the sponge (which was rotten) down to the hard, shapped backer. i went to a local material shop to buy some new material of a suitable colour and glued it to the backer using the evo stick contact glue in the red tin. it stuck really well, dont look to bad either. if you can accept the odd wrinkle around the sun visor area. all in all cost me about £20 and a whole saturday to do it, big improvement on the tent look inside and it was cheap!
i would recomend if your head liner is knackerd, give it a go. it can only go better than it is now, there's nothing to loose.