As this is a common step to take for several repairs on a Copal motor, I thought let’s dedicate a separate post to it. There are probably multiple ways to do this and this is just my take on the task, so please add any advice you can give 
How to open the gearbox
First you need to remove the cover. There are different versions of that. Some are easy and some deserve a whole thread on their own, so for now I conveniently skip this step
Next step is to remove the outer plate from the rivets/posts. To support the motor while working on it and to avoid damaging the rotor I use a wooden block in which I made a hole just big enough to accommodate the rotor. Using pliers I slightly lift the outer plate (not grabbing the post, only lifting) and then using an awl press down on the rivet.
The reason I push the rivet through the plate and not just pull the two plates apart is that I want to avoid separating the post on the other end (that happened to me once).
Before I used to also drill the rivets. That makes it easier to separate the plate, but I found it is not really needed, so now I don’t do that anymore.
How to reassemble the gearbox
Before closing the gearbox I first use the awl to slightly widen and taper the three holes from the inside. This will make it easier to press the plate back on the rivets. Next I position the plate on the rivets and make sure all axis are aligned with the holes in the other plate. This is hard to see, but you should not feel any abnormal resistance. A bit of wiggling may help to let the axis find their way into the holes. Then it is just a matter of pressing the plate at each of the three rivets until it snaps onto it.
Usually it snaps back on tightly and no further action is needed. If not, a little hit with a hammer and a centerpoint can fix it.
How to open the gearbox
First you need to remove the cover. There are different versions of that. Some are easy and some deserve a whole thread on their own, so for now I conveniently skip this step
Next step is to remove the outer plate from the rivets/posts. To support the motor while working on it and to avoid damaging the rotor I use a wooden block in which I made a hole just big enough to accommodate the rotor. Using pliers I slightly lift the outer plate (not grabbing the post, only lifting) and then using an awl press down on the rivet.
The reason I push the rivet through the plate and not just pull the two plates apart is that I want to avoid separating the post on the other end (that happened to me once).
Before I used to also drill the rivets. That makes it easier to separate the plate, but I found it is not really needed, so now I don’t do that anymore.
How to reassemble the gearbox
Before closing the gearbox I first use the awl to slightly widen and taper the three holes from the inside. This will make it easier to press the plate back on the rivets. Next I position the plate on the rivets and make sure all axis are aligned with the holes in the other plate. This is hard to see, but you should not feel any abnormal resistance. A bit of wiggling may help to let the axis find their way into the holes. Then it is just a matter of pressing the plate at each of the three rivets until it snaps onto it.
Usually it snaps back on tightly and no further action is needed. If not, a little hit with a hammer and a centerpoint can fix it.