One question is: What is the goal?
If getting some milling capabilities for the lathe is the main goal, I would probably just go for something similar to above shown bar stock design.
A good option to replace the dove tail would be some linear rails. They are not that expensive anymore.
If the goal is a good looking restored vintage machine, bar stock and linear rails would not be a good solution.
A toolspindle for the attachment and some dividing option for the head stock make it a 4 Axis machine. Perfect for milling gears and such.
I think I need to make a milling attachment for the lathe on the weekend.