[Plugin: Theme My Login] CUstom user fields in email

If I want to display ‘user_meta’ fields in the email to be sent one option is to directly modify the custom-email.php plugin file which I have been able to do by modifying replace_vars() function.
Is there any other way to do it such that I do not break the code when plugin is updated?
This question possibly can be answered by answering ‘can a plugin function be overriden in theme functions.php’? [I think not but just wanted to be sure] or is there any alternative

There is also a related question on cimy and TML posted by me – will be great if someone can answer that

http://ww.wp.xz.cn/extend/plugins/theme-my-login/