It is possible to drill holes in the standard SI24 carb air filter. This allows the carb to suck air more freely and, combined with correct jetting reading the spark plug

The reason that the holes aren't there on our scooters in the first place seems to be that the US and Australian markets were provided with detuned carbs (someone from Europe please confirm). This is just a guess and the fact that old Pinasco installation instructions talk about drilling a hole, suggests that "holeless" carbs were also sold in Italy in the past. This could also be a Cosa airfilter.

Dyno results show that the having a this hole provides a pretty big increase in power and at the cost of a main jet, is probably the best value for money performance enhancement you can get. See Air Filter Hole and Sito Plus

Here are is a picture showing the difference in air filters and the approximate size of the holes. You can use a drill to match these holes in your original filter, taking great care not to leave swarf in the filter.