Cast into the crankcase mouth of most two strokes are crankcase transfers. These allow the gasoline/air mixture to get out to the small passageways along the side of the barrel, which are called barrel transfers. These transfers travel up the outside of the barrel and become small windows through the barrel wall, called ports.
Most 60's bikes have only two crankcase transfers which are located directly across from each other. Above is a shot of a VBA with the barrel and piston removed, but the crank still in place. The two small holes just before the transfers allow gasoline/oil to lubricate the main bearings.
The more crankcase transfers available, the more ability you will have to get more gasoline/air up into the combustion chamber. Vespa realized this and added a third transfer in the 70s to the 150 Sprint Veloce, 125 GTR, Primavera ET3, and then all bikes after the Rally 200. The T5 increased to five transfers, but was the only Vespa to take advantage of this configuration.
Tuning a two transfer Vespa engine is a little limiting because there is not much gasket face available to enlarge the transfers. If you enlarge the transfers beyond the gasket mating face the engine will not get enough primary compression and will run very badly, if at all.
So basically if you are trying to create a fast Vespa start with a casing that allows the most amount of ports. Having said this it is not unheard of to alter the casing mouth if you know what you are doing. Above is a shot of a typical two transfer Vespa engine that a guy in Austria sent me. I lost his e-mail so I was unable to thank him or hear how the project went. He added aluminum where the red arrows are noted which allowed a larger gasket face and the ability to enlarge the standard twin ports.
It also allowed him to add a third port where there was previously no room, and probably use a much more up to date three port kit on an older motor. If anyone knows this guy please send me an e-mail so I can hear how it went.
For more info on three port kit transfer modifications, see the Malossi 210 porting page.