Hi Janpan,
You can keep a copy of your completed tournaments in a separate location. I've run a league for seven years and organize it like this:
Main League Folder
Season # Folder
Tournament 1 Folder
Tournament 1.tdf
Additional files (sign-in sheet, post-tournament results, etc.)
Tournament 2, 3, 4 (etc.) Folder
Completed Tournaments
Tournament 1-Complete.tdf
Tournament 2-Complete.tdf
etc.
After each game, save a copy of the tournament into the "Completed Tournaments" folder. When you run your league stats, you can point it at this folder and maintain the integrity of the rest of your season.
Hope this helps,
Rainier