Several observations:

Don't even think about using one of the wheel pilots. They don't have enough
oomph for a GB 42.

I assume that you have hydraulic steering with a hydraulic pump drive for
the existing autopilot. That pump if still good can be driven by any modern
autopilot. All you need to buy is the "brain" and the control head.

You may have cable steering and I don't know how this is set up for an
autopilot. Perhaps a hydraulic actuator drives the steering gear. A modern
autopilot "brain" should drive the existing actuator just as well.

I have used Raymarine and Simrad autopilots. Both work well. Make sure that
you get one with enough drive capacity. Raymarine has S1, S2 and S3
"brains". The largest can source 40 amps to the hydraulic pump. I suspect
that you will need the largest one.

David