I certainly appreciate the enthusiasm. However, changing core files is not the right way to go about it. That puts you in a position of having to update your hack everytime you update the plugin (or worse - not updating at all).
The plugin has a framework to accommodate customization. If you are needing customizations in the forms, the best way is to use the filter hooks to programatically filter the generated output and replace or insert elements that way (such as using php str_replace function). For the registration form, this would be wpmem_register_form. (A full list of filters here)
That does require some php skill, but it is the best approach. For those that lack the skillset or for much more complex customization, there is the option of pluggable functions. Much like WordPress itself, the plugin has functions that are pluggable.
A pluggable function can be replaced with your own version of the function. WP-Members looks to see if you have created a file in the plugin root directory called wp-members-pluggable.php and if you have, it will load it. Recreate the functions in this file and those will load instead of the plugin's main functions. (Note: this still requires you to pay attention to changes when the plugin updates to maintain compatibility.) There is a list of functions that are pluggable here. See this post on WP-Members pluggable functions.
For more information on how to customize the plugin's forms, see this section of the User Guide.