Hi Tomasz,
I'm not sure if it will get everything you need, but the "Forum Search - Advanced dialog" web part allows you to specify only the subject in the searched fields. As far as the results, you should be able to use the "Forum search results" web part to specify your specific transformation.
For both, you probably will need to copy the web part and add your custom code. For the search field, you can look at the ~/CMSWebparts/Forums/Search/ForumExtendedSearchDialog.ascx file to see how you can specify the options to display and how the search action is actually processed. From there, you should be able to build out your specific requirements.
For the index, the above web part actually uses SQL Search, not Smart Search. Because of that, you will not need to build an index as the calls will be made directly against the DB. It's not as efficient of a search as Smart Search, but gives you a lot of control over the actual query execution.