If you decide to use a big bore carb, you will have to replace the restricted manifold with an unrestricted one otherwise the you will see no benefit from the larger carb since its venturi size is actually reduced to the diameter of the restricted manifold.

This manifold will allow you to fit and see the full potential of any Dellorto carb up to 21mm. A derestricted manifold will give better acceleration and better top speed !