Create a datum curve where you can vary the diameter of each end independently and vary the distance between centers.
Create a swept solid that references the curve in the cross-sectional shape of your belt.
Assemble your belt and change its dimensions to match the sprockets.
I wouldn't worry about the ribs in the timing belt. Its like threads on a bolt. But if this is your concern, create a datum point on the curve and pattern it along the curve. Then create a cut across the belt, referencing the point and curve only. Reference pattern the cut. If it won't reference pattern, you chose a static reference.