Not truly a small change is needed, but still acceptable one, and it could be based on the solution which is provided for the login form. I have used the same function, only modified it a bit, so you may try the following:
1. Edit bb_plugins.php, below the file before the closing ?> paste:
/* Form's Submission on entering Topic's title */
/* --Form's Submission on entering Topic's title */
2. Edit templates/tools_edit_topic_title.html
, paste the following new fragments highlighted in bold:
3. This all would mean the allowance to submit the form using the Enter key when it's focused in the Topic's Title text field.
Let me know if it worked on your end (on mine, it's all OK).