I can almost picture it. Unfortunately, we can't see it and any specific advise might not be appropriate. Your description is good but it could use a few more details. You have beams with tounges and posts with grooves. Are you saying that the walls have gone out and need to come back in? Also, what are the cross timber supports you mentioned? Are they disconnected? What are they supposed to connect to? This stuff is a little too critical to guess at. What did the plowing do? Did it push the bottom of the posts in? Did it wrack the entire structure out of square? If the roof's spreading forces have only the counter of the post/beam connection then you may have to jack up the roof while pulling the building back together. This is pretty major stuff and you need pretty major equipment. Once everything has taken a "set" over the years, it can be extremely difficult to get things back correctly and the forces needed usually far exceed simple "come-alongs" and common bottle jacks. I would not catagorize this as a DIY project. If you mess up you can get squished.