Yes, you have to use the whole part going to engine side from old (OEM) electric motor, with ball bearing, oil seal and brushes. Because the axle of the 11 teeth motor is thinner the oil seal inner hole is smaller, not fitting on 12-teeth axle. The oil seal from 11-teeth axle will brake turning the axle, it's not suitable for 12-teeth axle.Hey Knob - when you assembled the 'mixed motor' did you do anything else to it other than put the 12T armature in the 11T case? How did it feel when you spun the shaft with your fingers? I'm just looking for reasons why it eventually showed error 26.
If needed - renew the brushes. Old ones may work well if they are not worn out. Also renew the ball bearing and oil seal if needed. In our case the electric motor had only 29000 km lifetime, so bearing, oil seal and brushes were good, no need to renew them.