The news comes from a site about models (as in scale models of, say, WWI fighter jets, not those scrawny goofballs on a catwalk), Model Graphix, which reports that a model creator, Logan Umemoto, went to the Italian Air Force Museum to shoot reference pictures of a Caproni Campini for the movie. According to the Model Graphix article, it's only after boarding this particular plane that Miyazaki will draw the Porco Rosso sequel, and no one knows if or when that'll happen.
I dunno how there could be a sequel anyway. Didn't everyone die at the end? Or, wait, that was Pigs in Space. No? Um...hm. Babe?
...Okay, okay, I give up!