2 martin-f: Do not look for bigger actuator. 18' you have will be more than enough to get maximum arc. I would say, even with 12' actuator you can tune your dish to the same arc. And with 24' arc will be exactly same. Only difference is how your system will withstand wind load. Of course, to use different sizes of actuator you need to adjust mounting-holes. Bigger actuator will move your antenna slower and vise versa. So, adjust your holes!
BTW I have 18' actuator on 1,5m antenna and it moves it from 30*W to 90*E (could adjust it for wider arc, but have some obstacles towards East and West). Same actuator worked some time on 1,8m antenna for same arc.