Jonathan -the Woo core files have changed since my last post. Having said that, I am now using the Striking Multiflex theme and you can now easily change the text within the theme settings and never worry about the core.
At the same time, I wanted to change the PayPal icon. Woo used to just call a PNG file but now it is going out to the PayPal site to retrieve the icon with this line in ../wp-content/plugins/woocommerce/includes/gateways/paypal/class-wc-gateway-paypal.php
$icon= ‘https://www.paypalobjects.com/webstatic/mktg/logo/AM_mc_vs_dc_ae.jpg’;
I replaced the above line with:
$icon =’../wp/wp-content/plugins/woocommerce/includes/gateways/paypal/assets/images/paypal.png’;
to use my own png.