All of the window glass on my car was in good shape-no cracks or scratches that I could see-but glass is heavy. Most race cars use Lexan clear plastic in place of glass-both for the weight savings and the obvious safety reasons. There are quite a few suppliers of racing body parts for Porsches, but the prices make you wince.(Well, they make ME wince-maybe you have money.) I visited a home-improvement store that was having a going-out-of-business sale and picked up several large sheets of 1/8" Lexan for only a few bucks-then used the original glass as templates to cut out my own new windows. After a few hours with a jigsaw, a Dremel, and some sandpaper, I had perfect replacement lightweight windows that would fit using the original-style rubber weatherstripping. (Plus, it'll make it much harder for vandals or thiefs to smash a window-ever try to break a 1/8" thick sheet of Lexan? Strong stuff.) I debated replacing the windshield the same way, but decided to use the original safety glass-I'm worried about the wipers scratching the Lexan-(and it'll probably be a lot easier to pass state inspection that way.) I ended up reducing the weight of the car by over 60 lbs by just replacing the side and rear glass with Lexan-and all the weight savings is up high in the car, thus potentially lowering the center of gravity a bit-great for autocrossing.
Some of the other methods I used to lower the weight of the car without sacrificing its usability as a daily driver: I drilled numerous holes in items like the window regulator arms, the wiper mechanism, the bumper brackets, and the lower portions of the heavy brass front door window frames(where they fit into the door shell). After stripping all of the factory undercoating off of the car so I could repair the rust damage, I applied a much thinner, lighter coat-just to protect the welds from rust. The undersides of the front fenders and rear quarter panels got a slightly thicker application of undercoating to try to protect the paint on the topside from being starred by rocks thrown up by the tires.
There are many other ways to reduce weight, such as using fiberglass (or even carbon fiber!) body panels, updating the suspension arms to the newer 911's aluminum rear control arms and front suspension crossmember, using aluminum brake calipers, etc-but they're all expensive. The original parts I'll be using are in such good shape I couldn't resist the urge to just use what I've got (at least to get the car finished and back on the road)-I can always upgrade to lighter parts later on if I want (need?) to-and can afford to. I did buy a set of Fuchs 16" alloys and aluminum lug nuts to reduce rotating, unsprung weight as much as possible-the wheels I spent many hours carefully wet sanding,masking and painting the center and spokes satin black and polishing the outer rims until they looked new(another tip-cheap masking tape is very, very hard to remove once it's gotten wet and then dried in place-(I reccomend using the "extended use" variety-and buy quality stuff like 3M-not the generic stuff-it's worth it!)