Obviously depends on what you want chap,
They'll all mate up to your Buddy Club 4-2-1 but to my knowledge your options are the following;
Keep the Mugen backbox and Mugen B-pipe.
Or
Get the J's racing 60R and source a OEM B-pipe, Spoon B-pipe or have a custom one made (Circa £150)
Or
Get the J's Racing 60RS which consists of the Backbox and B-pipe.