I've also been wracking my brain if I've read Kidnapped or not - the prota's names sounded very familiar and I thought I'm remembering some childhood exposition to it, but I don't have a phyiscal or digital copy nor did any of the cover images I found ring a bell (usually a decent means to help my memory up). But I just came across the info that there was a German YA series back in the late 70s I doubtlessly have seen.
These kind of series were awesome: one of Germany's major TV stations produced these really well-made annual four-parters, broadcast in the weeks before Christmas, which were kind of a staple for every child and many adults, and they covered several classics in that way - Treasure Island, too, and a few others I can't remember, but most about the 17th and 18th century and adventure-themed. I just found the list and realise I learned abot quite a few classics through these. I really need to see if I can find a few of them online somewhere; they were slow-paced and, compared to today's standards, often somewhat awkward, but even so well-told and well-made entertainment.
*dowloads another couple of books from gutenberg.org*