in tiki-register.php, look for mail(.
You will find
mail($_REQUEST%22email%22, tra('Your Tiki information registration'),$mail_data,"From: $sender_email\r\nContent-type: text/plain;charset=utf-8\r\n");
Replace tra('Your Tiki information registration') by your_subject.
Or if you are in a multilingual env, you will have change only the string and add the translation in the language.php file.