Kinetics licensed much of the technology that is in the Kinetics from Piaggio who also makes Vespa Mopeds. So the magnum is similar to a Vespa Grande in the drive train.
I specialize in Vespa mopeds so Kinetics owners in my area (San Francisco Bay Area) have been coming to my shop for repairs. I have had 2 Magnums in the last month both with the rear wheel locked up due the springs on clutch starter shoes falling out.. One of them also had a frayed belt like you described. I think what is causing the frayed belts is the belt itsself is too wide and will not go far enough down in the rear pulley and rides high up on the outer edge of the pulley, rubbing on the edge of the pulley thus causing the fraying.
I called Cosmopolitian Motors(the distributor for Kinetics) and they sent me a replacement belt that ended up having the same problem. Kinetics seems not to have used the same quality as found in the Vespa, for example the rear pulley on the Kinetics looks like it may be a slightly lighter gauge metal than the pulley off a Vespa Grande. This could cause the pulley to become bent and then cause the belt to ride up to the edge.
What I recommend is you try to find a belt that is the correct length but slightly narrow than the original belt. This will allow the new belt to seat better further down in the pulley and perhaps solve the fraying problem.
You can also try to get a Grande belt from Fabio at the Vespa Super Shop in San Diego or email me and I'll order you one.
Anyway, once you get the Magnums running I was quite impressed with their performance.
P.S. I ordered some extra clutch starter shoe springs so if you need any of those please email me for pricing.
Good Luck and Happy Trails