The ports are the same size..its just the throats that are opened out

Janspeed used to make a 4 branch, maybe still do altho I know they have just stopped the specific 16i exhaust.

Ashley also make one ( or did )

Camskill and Quicksilver also make them as well as ss reproductions of the original full system


Your best bet is a Janspeed, there's loads floating around usually