What I'd suggest is to follow the practice of Twain's own time. Twain was very fond of using a favorite profanity which cdn't be printed in books and papers in his day and so was indicated by an initial followed by a dash: d---.* Why not replace the offensive word in HUCK FINN with N+dash, just as news reports do as standard modern practice? Maybe someday we'd actually get to the point where a reader might genuinely not know what word this stood for. And that would be a happy day.
Even better would be to leave the text alone. If we're to assume that readers Huck's own age can't handle the use of racist language, wait until they're a little older to let them have the real book, not some watered-down version that distorts the reality the author was attempting to convey. Racism was ugly and to neutralize the ugliness of his portrayal of it distorts the picture. Twain famously maintained that the importance of using just the right word was the difference between lightning and a lightning bug; he deserves better than to be abandoned to the Bowdlers of the world.
*this particular taboo was not broken until a half-century or so after HUCK FINN, in Clark Gable's closing line in GONE WITH THE WIND.
The Day's Canto (Canto 5)
. . . on the barb of time.
The fire? always, and the vision always,
Ear dull, perhaps, with the vision, flitting
And fading at will . . .
--Ezra Pound [circa 1930]
UPDATE (Friday Jan. 7th)
I've turned off the comments, since several of the posters used the very word I'd carefully avoided. I'm simply not comfortable having what is, after all, a racist slur included in anything that has my name attached to, like this blog, whatever the context. I apologize to the posters, who to a man were thoughtful and well-informed; I hope they'll repeat their points on their own blogs, which I'd happily link to from here.
Also, after I'd posted, it occurred to me that I'd wound up agreeing with Tolkien's point of view, which probably won't surprise anyone who knows me. When discussing in OFS whether some of the more disturbing of the Brothers Grimm's tale shd be re-written to avoid exposing children to things like cannibalism, parents murdering their own children, & the like, Tolkien can down firmly on the side of no: keep the tales as they are, and if they're too strong for today's audience then hold them back until they're ready for them (cf. p. 48 of the Fleiger-Anderson edition, esp. the footnote at the bottom of the page). --JDR 1/7-11.