I do not know the size of the original Wien/BSR drive belt but as per my attached Thread link I used a spare flywheel flat belt from my Akia GX-4000D Reel to Reel recorder which fits and works well on my Wien.
https://www.vintage-radio.net/forum/...d.php?t=174845
My donor belt measures around 1.3mm thick, 6.5mm high and has an ID of 110mm.
My original Wien belt had turned to goo so difficut to tell but some of the remnants looked slimmer than 1.3mm
The absolute dimensions are not overly critical.
David