Sounds like a great title for a book or film, but it isn’t (yet) as for as I know.

Wow, as spelt, rhymes with how, now and cow, and that’s fine! Everyone knows where they stand. It does not rhyme with low, mow or tow, although, if it did, that is the sound I want to express.
It can’t remain as wow and have two different sounds because no one would know which one you were writing, albeit they have identical meaning.
Some ow ending words have both sounds, such as bow, row and sow, but the context makes it clear which sound is appropriate. This would not work for wow.
People do not write woe in order to get the correct sound across because that expresses sorrow or dismay, which is not the feeling they are trying to convey. I have on occasion seen woah used, but everyone knows that’s Horse language for stop (just as the double-click in the side of the mouth is Horse language for go). If woah is used, the recipient of the comment may think you want them to stop doing what they are doing.
With all the above in mind, I have decided to go for wo! It rhymes with go, no and so and is different enough to not be confused with woe or woah. (Wo is an archaic variant of woe, but who knew!)

It’s now going to be my new word meaning wow, but pronounced like woe
So, if I write wo you’ll know!