While the built in search is fairly decent, it's not exactly the best with story tags or overly common words. If you're finding the built in search isn't working for you, you can use the google site search, which is just a way of limited google to a specific website.
To do that you would add "site:https://thechangingmirror.com/phpbb/" to your query, and that will only return results from the forums.
Some example queries:
Female to Male : https://www.google.com/search?q=site%3A ... pbb%2F+F2M
Weight Gain: https://www.google.com/search?q=site%3A ... hpbb%2F+WG
IQ changes: https://www.google.com/search?q=site%3A ... hpbb%2F+IQ