Be careful of the difference between stall torque and rated torque. I got burned by this last year when I ordered new motors for my EC300. I was using motors with a rated torque of 277oz and thought that ones with a stall torque of 500oz would be better. It turned out that they had way less power and were pretty much a waste of $80. When I asked Robot Shop about the different ratings I was told that the motors measured at rated torque would stall at roughly 3 times that number...I'm not sure if that is correct though. These
http://www.robotshop.com/ca/en/cytro...gearmotor.html are what I use in my EC300 and 955K. They are on the slow side at 17rpm @ 12v and I am sure your sprockets will be smaller than mine slowing it further. I would recommend something faster for sure and possibly more powerful as well unless you were to up the voltage. These motors have plenty of power in the 19lb 955K but are way under powered for the 50lb excavator.
Reg