Is there any interest in the old VW rally cars?

Sure, if it was plastic maybe not much weight to worry about. But there would.probably be a fair bit of weight added for the associated mechanism, plus the strengthening frame around the hole to keep it square.

the std sliding VW sunroof is made of metal, plus the metal sliding mech attached to it, plus the huge metal cassette tape it all sits in. All very heavy, and the last thing you want in a Motorsport vehicle is unnecessary weight. Aftermarket VW sunroof is made of glass, so again heavy