Frank,
While a valid solution, you will find them quite expensive if purchasing new.
Go to <A target="_blank" HREF=http://www.mcmaster.com/>mcmaster carr</A> and search for "Electromechanical Actuators"
For a 500# force, 12" stroke $322!/w3tcompact/icons/eyes.gif
For the $$$$ you may want to rig up a hydraulic pump & use hydraulic cylinders. A Hydraulic cylinder the same size would cost about $70 new & have more force. I"ve heard of power steering pumps being used for lawn & garden tractor hydraulics.
How's the Cub running after the rebuild?
jrlichina, the links didn't work for me either. Maybe you have a cheaper source.