The original motor D6D05 is not manufactured anymore, however it is possible to install a replacement that you can find here: https://www.partsfortreadmill.com/kettler-d6d05.html
Let's see now a possible problem with the installation: Error 91 or 92. On most of Kettler treadmills these errors are related to speed sensor.
Consequently the treadmill starts and then stops
By better fitting the sensor, the problem will be solved