I'll say you have a treasure!
The belts are easily bought online and defintely check out the treadleon website as other posters have suggested.
the cabinet is beautiful and I know I have seen information online on how to care for those as well.
I have a red eye and my cabinet got messed up(WET) when we moved and I had it in storage. I however picked up a cabinet in good shape at a yard sale for 10 bucks. that was the cabinet only.
YEP you got a treasure! A usable treasure! And it was actually your grandma's That is great
think of when the power is out due to a snowstorm and you can be sitting by the fireplace,sewing and quilting away.