Hi Hoolee99 - The folks in the RC world have what you need. The servos used in the commercial throttle controls are from the RC (radio controlled) world. A really good radio control hobby shop can help. I know of one here in Spokane that can do this. PM me if you need further info.
For my money I'd still go with a unit manufactured for this purpose. iTroll, Troll Master, and Control King all make units to do this. The advantage among many is the ability to touch a button and return to idle when a fish is on and then resume the same troll speed once the fish is in the boat.
signfishin Alan