Looking at script.js, it looks like we may be able to fool that function into thinking you're still using Windows-1255.
Try this and see what happens...
if (smf_charset == 'UTF-8')
Add before that
smf_charset = 'Windows-1255';
That should fool it for now, but again, I can't guarantee this will work, so let me know if it doesn't and we'll figure something else out.