If it's a modern novel, you certainly won't make any money off of it (copyright law being what it is), but if you look around, you might find a nice fan fiction website where you can connect with other readers and writers who will appreciate your more satisfying conclusion.
Ulterior motiveOr you might discover why the author ended it that way in the first place. Your new ending could prove unrealistic. Or it could leave too many subplots unresolved. Or it could stall the psychological development of the protagonist. You might discover that the original conclusion is the only ending that makes sense.
Regardless, this exercise will not only get you playing around with words and contemplating story structure, character development, and reader expectations, but it will give you a closer understanding of the novel in question.
So for Vonnegut's sake, start with a novel you actually like.
WARNING: TIME SUCK If you want to have some fun with rewritten (movie) endings, hop on over to HowItShouldHaveEnded.com and watch some videos. A waste time, yes, but an entertaining waste.