Try Bintel, they are an Agent for JMI,who make alot of electric focus motors. They adapt onto your existing focuser. There is also Moonlite and Feathertouch. For full automation, try Robo Focus.
Try this...
I use a robofocus for both my telescopes. When you purchase this it comes with a custom adapter to fit the telescope focuser on your telescope.. It is not cheap at about A$500 but produces the desired function of providing very accurate fine control of the focus including fully automated focusing and temprature compensation which is almost essential once you have a fast scope.
The Skywatcher unit is purely an electric motor attached to the focuser without any of the smarts but does make focusing a lot easier and at the price is a very good option. It is not capable of computer control without first purchasing a FUSB from Shoe string astronomy.
Once you have had an electric focuser and in particular a robofocus or the Starlight equivelant you will never go back to manual focusing again.
The skywatcher one will not fit the Megrez 90 focuser, the coupler is too big for the focus shaft.. What you could do however is put a pulley on the SW focus motor shaft, and run a tight rubber belt around the pulley, then up and around the 10:1 shaft... As Doug said, coupled with the Shoestring FCUSB controller, it will give you PC control, and automated focus is possible.
The Robofocus setup is the best way to go if you don't want to change the focuser. If putting a new focuser on the scope doesn't worry you, the feathertouch focusers with either their own Digital Focus System or Robofocus is about the be all and end all of quality automated focusing..
