The following how-to was borrowed from Vespa-ciao.nl with permission.
Written By: Herr Flick
Pictures by: Tonni
Dutch to English translation by: FonS
The air inlet on a Vespa case can be drilled out to 13mm with a drill press. A 1/2 inch drill bit is 12.7 mm and will be 1000 times easier to find in the US.
Put a well fitting hose clamp around the inlet (to prevent tearing while you’re drilling).
The drill press with 13mm or 1/2 inch drill bit.
It’s best to remove the stay-bolts from the crankcase, so you can put it upright for drilling. Measured from the end of the inlet pipe, you can only drill up to 18mm deep. If you drill any deeper, it might cause a hole in the crankcase. (Ruin the Case!!!)
This is how it looks after drilling.
Now it’s time to file the inlet. Always file up and down, never left to right! How far up and down you go, depends on your setup (cylinder and so on) or just how far you want to go. You can file to very near the edges of the inlet area (stay 2-3mm away from the edge, to prevent leaking).
When the filing is done, you need to smooth out the surface (you can use a grinding tool, but always work carefully, the material is quite thin).
When everything is done and smoothly finished, it should look like this (unfortunately it’s a blurry picture).