I am not familiar with your particular items, but when I need an uncommon specialized machine part, I often look for it at Grainger. Doing a search for "Unienti B463MXL" there, I don't see an exact match, but it sends me to a page with a huge selection of belts, broken down by types, and with specifications shown in the lists:

Grainger belts

That probably won't quickly and easily get you exactly what you need, but if nobody posts anything else, you might be able to find something close enough to work. I doubt that a special belt was created for that aircraft, so they probably(?) designed it to use an off-the-shelf item. I'm not certain about that, though.

I hope that helps.