I don't think the thickness of the rubber matters, but the narrow width of a motorcycle tube might make it difficult to get a decent width belt... dunno, try it.
The car tubes I sourced had mold lines around the circumference making it easy to hand cut parallel widths... that's the part that I think is probably the most difficult to achieve, if aint cut straight - it aint gonna run straight.
The curvature is somewhat retained, but with a stretch fit, a lot of it is lost. Probably still have to make an angled bearing'd roller to maintain V shape.