Where is a fully functioning version of the mp_product.php that can be modified?

We would like to override the behaviour of the product detail page. We understand that we do this by dropping in a file called "mp_product.php". We can do this and it works. Unfortunately we have no idea what is meant to be in one of these files to implement all of the basic functionality, such as price, quantity etc.

Where can we get a fully functioning version of this file, and all the other files that define the behaviour of the store?

Thank you for your help,
Frances