1. Scrape it. Plenty of options here.
   2. set up a new forum. 
      I think the current state of the art is Discourse but I could be wrong.
   3. automate the recreation of posts and threads with some backend script
      (will depend on which sw you picked). 
      On each post, add a link to the original.
   4. Tell everyone about your superduper clone, move the old-timers over.
   5. ...
   6. Profit.