In place of salt mackerel, regular saltfish can be used as well as smoked fish such as de-boned red herring. The flavour of the dish will be different based on the type of cured fish used.
Chadon beni is also known as culantro. If you cannot find this herb, use fresh coriander/cilantro as a substitute, however, you will need to use about 1/2 cup of roughly chopped cilantro.