I assume you mean the black stuff covering the whole floor pan. I assume so, and it is a bitumen based sound deadening - ie like Dynamat.
You can remove it pretty easily using a heat gun and a scraper. When it is out, check for rust as it can trap any water that comes down in. Given you have a well-galvanised Carrera this shouldn't be a problem, but it doesn't hurt to check.
Replacing it is a piece of cake. Basically clean up the residue as best you can, then peel and stick the new sheets, using a heat gun (or even a hair dryer) to help them conform to the shapes of the floor pan. Make sure to eliminate air bubbles.
In terms of what to use, Dynamat is an option, but expensive. I used a more generic product made by Bostik but probably not available in the US (and about half the price). Someone else can probably give you a recommendation.
I would recommend replacing the fibre underlay stuff too - it is very very cheap at your local upholstery shop.
I did a quick search on "sound deadening" and it came up with this link, which is probably as good as any:
http://www.projecttransam.com/projects10.htm
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Cameron Baudinet
1975 911S