There is no need to modify any files to achieve the checkbox selected by default.

Find in the original create_acount.php file:
tep_draw_checkbox_field('newsletter', '1')

And set the third parameter to true (it is set to false by default in the html_output.php)... in other words, replace the previous with:
tep_draw_checkbox_field('newsletter', '1',true)

Is that simple.

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