On my Bruder D4 conversion, I used old servo's. The potentiometers were fried, but the motors were good. I just removed all the excess junk from the case, leaving ONLY the motor, then removed the limiting pin from the gears. This method requires a Sabertooth controler (OR similar ESC) to control them.
On the Bruder D11 conversion that I will be starting sometime, I am planning to use these.
https://www.ebay.com/itm/SALE-0-6-12...72.m2749.l2649