What is so different about this film, what makes it such a great reflection of the book on screen - is that the director, it seems, did not aim to make a perfect comedy, or a perfect adventure film, or a perfect family film, and so he did not try to fit the story into some sort of industrial template to make it financially successful. In the USSR, movies usually weren't produced to make cash.
Instead, what he did was pure art. He somehow managed to catch the atmosphere of a small American town on the bank of Missouri, the feeling of life there viewed through the eyes of a little boy. He certainly takes his time picturing it, but the result is very convincing and emotional. It does not impress you with dramatic action and effects. But it leaves a very warm aftertaste. And, yes, it does respect the original story.
Sadly enough, poor preservation of Soviet film material and general situation with the cinema industry in modern Russia will hardly ever allow contemporary movie lovers to see it in good quality.