After some research and without actually opening my engine yet, I found out that the transmission belt type which is described as 'Cinghia Dentata 124 L' in the manual, means that this is a '124L' type of belt, and not 124 teeth.
This belot is actually quite easy to find. I bought a Gates in 0.5" (inch) width, they are also available in 0.75" and 1" - extract of the original manual is below:
Here are some pictures of the belt I bought for a few US $, Send me pics of your Zanetti Bicizeta by sending me an email at zanettic50@gmail.com