A hydro works linear actuator (model 8489098) is what we are looking at using this year. After opening it up the motor supplied it almost identical to a bag in sizing. The bolt holes are slightly different so it's not quite plug and play but for the price it's pretty darn good. Throw a flat on the bag motor shaft and drill 2 new mounting holes and you're set.
http://www.princessauto.com/en/detai...or/A-p8489098e