You need to look a bit harder! Tomaselli, Amal, Doherty levers are all available. Not seen any Maguras yet but they may be out there.
They are indeed, I've got boxes of them that fit older British bikes, but I haven't seen any reach adjustable ones that are designed to fit current Royal Enfield Interceptors, only the generic TEC or Pazzo style and their knock offs.
If you have a source for anything styled like the levers I mentioned that are reach adjustable and designed to fit the current Interceptors please post it. I'd buy a set in a New York minute.