Original logo file
 

Is an original logo file (photoshop layers) available to easily customize the header logo to match the rest of the header without a lot of trouble?

Do you mean so that you can copy the pattern or to use it as a guide? If so, I don't think so...

You can, however, right click to find properties and it will list the width and height in pixels. Then create yours to match that size. You can also find which logo it is you need to replace this way (file name and location).

Here's one I made just for you

http://www.shop-for-gifts.co.uk/x-cart_logo.psd

Right click and save target as..