My website to transform latex into ANS-katex can be found here.

I use this website as follows. I copy-paste my latex code in the large text-field. By clicking the button, in this latex code $$..$$ is replaced by \[..\], $..$ is replaced by $..$, and tildes outside of these math environments are replaced by spaces. Also, hard returns are removed. For most of my Latex code, this suffices. The result is then copied to the clipboard. If you would want/need anything else, please let me know.
I am now also experimenting with exporting the katex-code from QTI-files. Click here.

You can also do this locally: For that, download this zip-file with all code, extract it, and open index.html in any modern browser.