I am looking for the best way to integrate this custom scroll code into a Saola file? If you launch the html file you will se it slows down the scroll when the page reaches the very top or very bottom.
Firstly, your code set and control scrolling of document.body tag, not the scene element. So you must set scene overflow to visible to remove the scene scrollbars.
Secondly, you must update your code to make sure the height of document.body is set correctly based on your content height. You content height can be gotten from position and size of the bottom-most element on your page (Group_1 element in your project):
var bottomMostElement = doc.getElement('Group_1').dom;
document.body.style.height = (bottomMostElement.offsetTop + bottomMostElement.offsetHeight) +'px';