Hi Sören,
you can´t use the 750 Gasket on a 1000 cc barrel, it has a much too small diameter. But you may use it as some punky-style ear-rings instead?
If you are in need of 1000cc Gaskets, ask Sepp Koch, he had some  last year, they are much better as the ones from the "Ebay-Gasket-Sets",
Ali