I've had the same problem and in my research have found that you have 3 choices re replacing the silencer.
1) replace with an original Alfa silencer
2) get a twin box conversion from Ragazzon, Supersprint or Alfatuner (Alfatuner was a very reasonable price I must add).
3) replace the original silencer with a Supersprint one (the left box from their twin box conversion kit)
Cheapest is 1, then 3, then 2. Original box is about £300 and the Supersprint box only (stainless) is £345.
The Supersprint complete conversion is pretty dear in comparison to the Alfatuner one.
Supersprint don't say the left box will replace the original with existing pipework but 159_kid did and said it went straight on. I'll be able to confirm that in a few days hopefully.
If you just want to replace the silencer asap but leave the option for the twin conversion later then I think you either get a standard silencer now and then throw it away when you have the money for a complete Alfatuner system OR get the Supersprint silencer now and then add the splitter pipe plus right silencer later. Unless someone has found any other options?
Hope that helps in your decision making.