If you decide to use a big bore carb, you will have to replace the restricted manifold with an unrestricted one. Otherwise the large carb will be useless since its venturi size is actually reduced to the diameter of the restricted manifold.

This Motoforce enlarged manifold will fit any Dellorto carb up to 21mm. A derestricted manifold will give better acceleration and better top speed.
This manifold has a slightly shortened inlet tract so for some scooters it may not be possible to still use the original airbox.

Motoforce MF15.14000