I think I would agree with that. In an era where children’s films, are most often animated or scifi/comic book stuff I found it a refreshing change. I haven’t read the books, but did grow up playing in small boats. I doubt it would have made it to the big screen without the espionage angle, and Ransom himself was allegedly a spy of some sort. I think if you watch a film like that and are getting too obsessed with the details like whether that particular model of plane existed you may have missed the point: its light entertainment not a historical reenactment.
But it’s just not necessary to mess about with the story by chucking in this daft spy business, there was enough mystery about what Jim Turner/Captain Flint, and the theft from his houseboat to keep the entertainment going, and the floatplane is a glaring inconsistency; there is never any reference in any of the books to aircraft, the whole point of them is the connection to sailing and adventure!
I have read the books, all of them, in fact I’m reading one of them at the moment, and still enjoying stories set in 1929 onwards, without any re-writing to encompass spurious modern subjects.
The 1974 film managed to keep Titty’s name intact, too, although the 1963 BBC series had Kitty instead of Titty, played by Susan George.
Titty was the nickname of Mavis Altounyan, one of the children of John Altounyan, who the Walkers were based on, the name coming from a children’s story by Joseph Jacobs, called Titty Mouse and Tatty Mouse, which is why Tatty is an acceptable substitute in the film.