And the cause? It looks like the fuel pump or the wiring to it, or a combination thereof:

The fire spread to the plastic rear fender and seat, as well as downward courtesy of fuel leaking/catching fire onto the battery box and rear ignition coil. The insurance company totalled it out, but I opted for a salvage title/buy-back. Although I am still awaiting the title paperwork and check, I rummaged through the "Great Heap" of parts I have amassed over the years and found a rear fender, a torn seat, a coil, and a fuel pump.

I still need to acquire some bits & pieces before I can slap it all back together, and the fuel pump I found leaks so it will need replacing (again), but I am content to have gotten this far without spending a dime yet.