About a year ago, I was given an iMac Indigo Summer 2001. In a matter of days of playing around with it, I became a hooked collector of old Macs.
One of the Indigo's speakers had a fully rotted-out speaker surround, and the other speaker was just starting to flake apart. I was able to take out the "eyeball" speaker holders, and outfit both with pretty close replacement speakers:
http://www.parts-exp...lProductDetails
They cost about $13 for a pair. Installation wasn't too difficult. After soldering on the wiring (I didn't wind up using the new lug connectors, since they weren't a perfect fit), I placed each new speaker inside its "eyeball". The mounting flanges on the replacement speaker aren't quite perfect, so I stabilized it by stuffing a piece of high-density foam between the back of the speaker magnet and the "forks" inside the "eyeball". Once the halves of the "eyeball" were screwed back together, the speaker was quite firmly mated to the front opening. And let me tell you, compared to speakers with missing or failing surrounds, they sound great in the iMac!
Given the cost of the replacement speakers above, I wouldn't try to repair the surrounds on the original units. But, I understand if you prefer all repairs to be as authentic and as-original-as-possible, you'll reject my replacement suggestion. At least you now have an alternative if you give up trying to re-surround / re-cone the original speakers.