Steve,

While they're not exactly the same as Becky, and Finn, there are a number of
"continuations" of Twain's works (which I am particularly interested in,
though haven't had much time to do anything with).  Here is my list, which
others can add to:

The Further Adventures of Huckleberry Finn, Matthews
The True Adventures of Huckleberry Finn, Seelye
Tom Sawyer Grows up, Wood

That's all I got.  I hope it helps, I'd love to hear what the others come up
with.

Michael MacBride
Minnesota State University, Mankato