This is pretty ingeniously done. The writing style isn't always a pitch-perfect Jane Austen imitation, but it's a pretty good match for Mansfield Park. The introduction sets up many critiques of the original that I almost universally agreed with, and the subsequent narrative addresses them well with a keen eye for incorporating as many details as possible from the original text (and some amusing allusions to other Austen novels).
Maybe I'm missing something from the original, but I just didn't enjoy it -- and this new, more satisfying, more fleshed-out ending made me like the story much better. Without ever having been a fan of Henry's in the original, here, I'm swayed and I buy his change of heart as a reformed rake.
The book starts very near the end of the original story, with Henry Crawford's encounter with Maria Rushworth, and paints a picture of a man who is at fault, but not as irredeemably (or even primarily) as suggested in the original. It picks up again with the chapter where Fanny receives the letter from Mary Crawford insisting that the scandalous rumors aren't true, and then takes us inside many of the conversations that are only alluded to or described in the past tense in the original. Fanny and Edmund get some much-needed character growth, and Henry and Mary get redemption, but slowly, and within a detailed narrative, instead of in a summary by a narrator. The alternate ending changes some of the facts to launch the new plot direction, and continues to diverge as time passes, but largely keeps the characters acting in ways that feel plausible. And -- this is among the hardest changes to go with, but also among the most satisfying -- Fanny actually makes a few decisions and takes some action!