Looks ain't everything; checked out that possibility when I first got my 1500A. Bevel gear output shaft way bigger on the 88, as is the input to the final drive. Still, that's not to say there isn't something out there that'd work; just takes some looking. After all, a 16" Nomad wheel can be made to fit the Vulcan 88...The 1996 Vulcan 88 I just fixed for a guy had a rear disc. Maybe the wheel and pedal and etc might fit our bike. Looks pretty close
The only reason that I asked would be for maintenance - changing a disk pad is a lot easier that a drum brake.If you search back though the threads, this question has come up at least two or three times in the three years that I have been frequenting this board. As Ccspinner says- anything is possible with enough money, time and skills. In the considered wisdom of the group, the consensus seemed to be that the project would be more trouble than it was worth.
You can lock up the rear wheel with the drum brake already, so why do you need a more powerful rear disc brake?
As a purely technical exercise, it might provide an interesting challenge to some mechanically talented individuals. But that is the only reason I would consider for doing it.