Update...I just got a brand-new Bet & Win 150 drive belt from the KYMCO dealer today. Here are the measures to the best of my ability (with just a little range...splitting hairs really): Length is 811-812mm, top width is 18.5-19mm, bottom tooth width is 13.5-14mm Not sure how to measure tooth angle exactly, but feel free to tell me how, if you know.
There is a Malossi X-Kevlar drive belt for the B & W 125/150 for sale on sip-scootershop.com that measures 811mm long, w 18.5mm wide, 9.5mm high/tall and 30° tooth angle. All measures of which are most similar to the size I got directly from the KYMCO dealer.
Hope this helps someone sometime!