What you have in your photo IS the actuator. Top Hydraulics is the place to get them rebuilt.
https://www.tophydraulicsinc.com/en/bmw/21-bmw-z4.html
You can get Shipkillers DIY instructions on removing and re-installing the top, and one on replacing the hinge assemblies (actuators).
http://www.bimmerfest.com/forums/sho...&postcount=114