Hello,
You can always clone the repository and switch to the tag version that you want. There was no readme at that time and I doubt anything that I wrote in the current readme would apply to that time; you will be alone if you follow that path.
However, I advise you to not even try: the world has moved on since that period. If there's really no way for you to update, you'd be better off installing the version of Mercurial from that time and do your thing on the command line/terminal.
I'm not in your shoes, but I can only see a long string of hurdles.
Good luck.