just change to your side easy , no need delete any text .
GO to : catalog/language/english/mail/
find " open cart , than change to your site name , then done .
OR
find
$_['text_powered']
in
catalog/language/english/common/footer.php
find
$_['text_new_powered']
in
catalog/language/english/mail/order.php
and change it.
to change the size and font go to
catalog/view/theme/yourtemplate/stylesheet/stylesheet.css
find #powered
