@Thaleef kind of done. Had to do this on chrome developer tools as i am on development store and to try it on your store i will need acess. You can add this to your base.css file. It could be some other file but i am unable to open it since i am on free trial and it does not let me edit code. Anyways this is the code.
.site-nav__dropdown .megamenu{
min-width: 50%;
left: 10%;
height: 500px; // In order to have the scrolling feature you have to set a height on megamenu. You can change it as per your liking.
overflow-y: scroll;
}
.site-nav__dropdown .megamenu .grid{
text-align: left; // i believe you can change this from the customizer, but this will work
}