I got the Dune Cat about 10yrs ago so I'm not the original owner but it doesn't appear to have been swapped out.
I guess that would mean you have the T88 belt. 5/8 Symmetrical. I wonder if that Horstman would work?
From what I gather, it's hard to find that old Comet T88 setup. I remember Mac got a bunch of Comet stuff at a meet, and I did get one of his driven units. You might shoot him a PM.
You could also try mine, as obviously no one needs it for their TAV Cat bikes. Not sure if the belt angle is the same, but it is otherwise suitable for a 5/8 belt. 5/8 Jack shaft. $20 plus shipping if you can use it.