If it is something like teleflex driving a tiller arm off the rudder, you could rig up a small autopilot. If it is to be driven off the wheel it's guner cost.
You don't mention to what extent the boat gets used. If you are a professional fisherman using the boat 4 or 5 days a week, I would suggest you need to consider an inboard system such as simrads AP11, Raymarines ST6001 or Silvas Nexus. The actuators are more varied and more robust.
Silva system would probably work out cheapest if cost is an issue.
If you've got hydraulic steering these systems are relatively easy to install. Mechanical systems require a bit more thought, but not a major problem.
I wouldn't recommend a wheelpilot for higher usage, but I'm sure someone here has taken one aroung the world!
Thanks for the replies - I am not professional fisherman, but do odd 8 hour passages. It is a teleflex system, not hydraulic. I still have my autohelm from yacht days (it was superb and good value). As the lever on rudder is well tucked away rigging it up as a system could be difficult. Regards Nic