I have throught about this too for a Garelli I had and there was one very good carb for this. It not only is the perfect size for the big bore kit but would be perrfect for almost any horizontal cylinder.
Just go to the local junkyard and find a Suzuki T125 twin. As a twin it has 2 65cc cylinders and that is the same as the big bore kit. It came with a down draft carb. PERFECT!
I never got a chance to do this as another project took over but here is a picture of the carb in question. At the local junkyard.