I have opened 4 or 5 vespa intakes using a professional mill and found that you can only safely open the throat of the intake to 12.7mm or the same as a 1/2" bit.
Any more and you are risking snapping off the intake from just the weight of the carb it is supporting.
Drilling down the throat is only half the battle as you cannot drill all the way through because as you near the inside of the case near the rotation patch, the bit will punch a hole through the side and create a leak.
The rest of the opening of the intake has to be done from the inside using a chainsaw file
You mention 10mm carb. I thought you had 12mm?