Most of the code you will want to modify is located in the thematic/library/extensions folder. Both of the things you are wanting to do are located in the actual content-extensions.php file in that folder.
As far as modifying the excerpt, that depends… on exactly what you want to do. The function you want to filter is thematic_content_init() and an example http://themeshaper.com/forums/topic/full-posts-on-category-pages can be located at that link, you might want to check the actual function though to see the full list, as things like tags, author, archive, would also need to be changed. If you wanted everything changed, just remove the conditional.