Thanks Brett.
I called Spartan to get their advice. They told me that (as I suspected) I probably wouldn't be able to use a gear puller, as the radiator is in the way. Of course, the motor can't be removed from the bracket with the fan attached. They confirmed my observation that there's only an Allen set screw holding the fan hub onto the keyed shaft of the hydraulic motor. They said that I should be able to (carefully) tap the hub off the end of the shaft without too much difficulty, once the set screw was loose. (We'll see about that!) They didn't seem to think that I'd have to bother disassembling the fan from the hub adapter (by remove the 6 bolts holding it on.) I guess that I could remove the set screw completely and squirt in some PB Blaster to help it along if it gets stubborn. They also emailed a drawing of the entire assembly so that I could see exactly what I'm dealing with.
Another good suggestion that was made by someone was to set a piece of cardboard between the fan and radiator, just in case that fan slips upon removal and to avoid possible damage to the radiator.
We'll see what happens when I get back down to the rig (still in storage) this weekend and try to get to work on it.