The easiest way I've found to make a completely custom scrollbar is... not easy.
The basic idea though, is you make a "Frame" that masks the outside of the scrollable area, and has basically a "cut-out" or "window" that shows the content you want to scroll. Then, make a background for the scroll area, preferably a tiled background image so you can dynamically scale it taller or shorter depending on your content requirements.
You then have to make a scroll bar thingy, and always set it's X position to a fixed number on the layout or layer and add the drag and drop behaviour to that.... then you simply set your scrollable tiled background Y position to be the same as the scroll bar thingy, with some additional math (the height of the scrollable area divided by the height of the scrollbar itself) then you do set scrollarea.Y to scrollbar.Y*(scrollarea.height/scrollbar.height) - I think xD
Adding content to the scrollable area can be tricky, depending what it is... but you can use sprite fonts, sprites, and whatever you want... position them according to the scroll area tiled background - make sure your "frame" is in the front of everything (z order top) and it should work pretty good!
I have to make a scroll area for another thing I'm messing with, so if I end up doing that today I will upload an example. Either way, hopefully my really bad explanation will help a little bit.
I should read more carefully... I just realised you wanted momentum in the scrolling... that is another problem all together. You could do it by having a dummy object that takes control of the "scroll thingy" if the user has previously scrolled, but let go of the mouse or stopped touching the screen. You would need a variable or something to track the "scroll amount" - maybe a number that goes up based on how far the user has scrolled, or if you can create some kind of velocity measurement based on a combination of distance and time of the scrolling action? Use this variable, possibly combined with LERP, to make the scrolling continue for a short while after the user releases the scroll thingy.
Yes... "thingy". I'm a very technical individual, as you can see.