I've been hacking on my own clone [1] for the past year with a WYSIWYG editor based on ProseMirror. Here's the demo page [2] for the math editor!
[1] https://github.com/benrbray/noteworthy (disclaimer: not ready for release -- hoping to polish it up by the end of the year)
[2] https://benrbray.com/prosemirror-math/ (disclaimer: the demo page is quite minimal -- many extra features, like Markdown syntax, copy/paste, etc. can be added through ProseMirror)