For a working shop it's always much cheaper both in time & money to buy a working machine than to retrofit. This holds especially true for a knee mill.
For home shop use where maintenance time isn't billed to overhead a retrofit can be a good solution. In the home shop environment the control doesn't need to be as polished as it would in a working shop where many different people might operate the machine. If the shop employs more than 9 people (as I recall) it wil fall under OSHA which imposes some more stringent requirements on machinery. These are mainly in the Emergency Stop circuits and the Start button circuits, many DIY retrofits fall short in these areas.
Turn key retrofits are available from several sources. Centroid and Analam come to mind though I haven't heard anything good about Analam's controls.
DIY retrofits are possible if there's a person who will have time to take charge of the process and see it through to the end. Sometimes finding this person is tough in a working shop.
It may be possible to replace the tape unit with an emulator to get the machine to work mostly as is. The trouble with this plan is this machine should have a B or BX control which is really obsolete, and running it as-is doesn't improve the control.