Jim,
As an avid RC hobbyist, and father of two children who just *happen* to like all
the toys I always wanted but never got (as a child), I can tell you that no
filler or glue will make the repair you describe.
Try this instead:
Find some stiff steel rod (a heavy needle or hat pin will probably work) and
with the gear in position on the motor shaft, drill an appropriately sized hole
completely through the gear and shaft perpendicular to the shaft.. Use a brand
new drill bit of better than average quality (Bullet (tm) by Black & Decker is a
good one for this. If the motor shaft is too hard,  you might need something
better). Go through the gear to the shaft, marking the shaft with the drill bit,
then remove the gear and finish drilling the shaft, trying hard to maintain the
angle of the hole in the shaft equal to that in the gear. Replace the gear on
the shaft and finish going the rest of the way through the gear. Rough up the
needle (or pin, or you could even use the drill bit, if you can break it off the
right length) with some coarse emery cloth or a file to give the epoxy some
purchase. Trim the pin to the correct length and epoxy (again, go for quality.
You probably won't get a second chance at fixing this part. JB Weld is a good
choice) it in place. Let it dry longer than recommended on the tube. Take your
time. Do the best job you can.

Good luck! 

Andy McKinley
75112.2101@compuserve.com
'72 Westy