Well here's how I'd do it.
Get an offcut of flat aluminium bar (say a few mm thinck) then by means of an undersized drill and rat tail files create a hole that fits the profile of your shaft perfectly.
Now using this as a template, get a piece of wooden dowel and file it down to make an exact copy of your shaft.
Now get a piece of wood whose thickness is about equal to your shaft length.
Cut a hole in this piece of wood a bit larger than the hole in your gear.
Find a piece of tin plate (such as the discarded top of a baked bean can and screw this down over the hole you just made.
Make a small hole in the centre and use this to screw into the end of your replica shaft.
Laying this flat on the bench you should now have a well with a wooden shaft sticking up in the centre.
Now finish off the baked beans, wash and clean the empty tin.
put it on a gas ring on the cooker and put in some scraps of die cast.
Once this has melted, pour it into that well that you've made and allow to cool.
Once set, drill, scrape, file or whatever it takes to get the moulded diecast out of your mould.
Push it onto the motor (ideally it wants to be a very tight fit, but it probably will be as die cast shrinks as it cools.
Clamp down the motor on a bench and power up the motor. Using files (or ideally some turning tools held against some kind of stop) turn down the adapter until it is perfectly round and just the right size to fit into your gear. Before you test it, however, you want your gear to be hot so keep it in the oven between tests.
Once it's a good tight fit, leave it until the gear cools. It will then tighten up onto the adapter and should be solid as a rock.
I do think, however that THOSE gears are much too large for that motor. They'll struggle to turn them with any worthwhile load on them. Just the power required to accellerat them from a standstill will cause some hefty strain on the motor.