Not familiar with the original story or the adaptation, but that does indeed sound like an excellent improvement on the original, in terms of making it a romance you can root for.

I linked the original story above; it just looks like a scan of the relevant fifteen pages of Welcome to the Monkey House. The adapation was streaming from Netflix as recently as last year, but Netflix is a fickle provider. I think my best recommendation at this point would be to check libraries.

(I suspect there would be an equally good way to improve on it by playing up the weird dubiousness of them only matching when they're not being themselves, but it would be a much less happy result, if thought-provoking.)

I think it's not even that I find Demme's a happier ending—I just like it better when it's clear that both parties are people with choice and change and interiority. I don't mean there's nothing of that in Vonnegut's original (and I'm open to being told I've been misreading the ending all these years), but the adaptation foregrounds it. It deepens without too much opening out. I think the results are a lot stronger.