Here is a link to the Olsen page for the 263E motor.
Note the E unit is positioned just like yours and, yes, the E unit can be reversed for application in different locomotive shell.
Can you provide a photo of the motor showing the brush plate side ? The motor looks clean. Do you know if the previous owner had serviced the armature, brushes and E unit? If so, we can assist you in testing the motor. If it has not been serviced and you need assistance to accomplish that task, we can assist with that as well. Once the motor is operating as intended, assistance in reassembly can be provided.
As a first step in reassembly, I would print out all five pages of the Olsen material BMORAN4 provided a link to as well as the page from the above link. With copies in hand, match your parts to the Olsen parts to make sure you have a complete set of parts. You will get somewhat of a feel for assembly as you undertake this task.
Here is another link. This is to a EBAY offering but some good photos are shown that could be helpful in reassembly.