Don't know about the US, but in Europe that wouldn't make any difference to the value, may even enhance, for the reasons that Richard stated earlier.Very true. But given what's been happening with Alfa values these days, personally I'd suggest keeping a car with Dellorto's original if at all possible.
I have not found a source for Dellorto parts here in North America, so GTA R's above comment is even more of a factor here. I am aware of several U.S. sources for Weber parts.I deal with many sets of Webers per year as a part of my living, and can get almost all parts next day from several UK sources, not AC or AH. Not so with Dellorto. Nothing wrong with the carb at all, but parts more difficult to source ,