Ade, I have no knowledge of the Quickturn controller, but don't under estimate the complexities of replacing it. Entirely possible of course but the shear volume of inputs and outputs on a complex CNC lathe is quite a challenge to deal with in an orderly fashion in the timescale that machining demands.
Remember that the first version of your lathe probably took a team of half a dozen several months implementing and integrating the control with the lathe, and they almost certainly had already 'done it before' on earlier models.
The huge advantage of replacing the controller is of course that everything is under your control with no issues of lack of information from long gone suppliers.
Example: if I had a paper copy of the ladder logic of the PLC on the Beaver, I'm certain I'd have found the issue with spindle orientation by now.