20 gauge shells are not commonly available in this country and they will be more expensive, probably incuring shipping costs.
Reloading is certainly possible.
Some short chambered 20 ga. guns were built heavily enough to be re-chambered, but even so will probably not withstand "high velocity" express loads for long , especially old wood stocks.
With equivalent shot charge and choke they will perform just as well as a 2.75" gun.
Just consider that most short chambered guns you come across in this country will be 75 years old and made for lighter charges than are generally thought necessary today. If that seems to be what you want, you can get a beautiful light fast handling gun - but not one also adaptable to heavy charges or steel shot for waterfowl.
Cheers,
Bob