I've actual put quite a bit of thought into this, as I have shaper syndrome, a condition in which you impulsively buy shapers and planers until you no longer have room for anything else in your shop, house or yard.
Here are my thoughts (and I am working on plans to convert a Lewis shaper to CNC right now)
Requirements:
1. CNC machines are meant to be set up, turned on and basically forgotten until you need to change the part. Shapers, especially the older ones require constant attention. Any CNC shaper that is more than a proof of concept will need to have either sealed bearings or automatic oiling for the ram moving parts. I've seen too many shapers and planers with ram ways scored because of lack of lubrication.
2. CNC machines need a tooling system that is easy to change out and consistent, which means some sort of quick change tooling system. I had planned on using a BXA lathe tool post with custom holders and carbide insert tools on my machine.
3. You don't need a clapper box. You also don't need the toolhead to rotate, in fact it should be pinned perpendicular to the table travel. You should be able to just program the tool to move a clearance distance away from the cut on the back stroke. You can also program it to cut any angle within the capability of the tools you have.
4. You don't need ballscrews. Some sort of backlash takeup would be nice on the toolhead, but force on that tool should only ever be in the upward direction, I thought about using some air springs for it to take that up. You should be able to get away with planning the cuts to not change directions and use backlash compensation. For that and a few other reasons that I will get to further down, LinuxCNC is my choice for control software.
5. You don't need encoders on the ram system. Some hall sensors and magnets or switches and a cam should be sufficient. Some adjust ability would be nice if you wanted to make sure you had the maximum time to move the tool in and out of the cut, although I won't know if this is needed until I see how fast I can accelerate the table and workhead. You could also compensate for this somewhat by using more lead in and lead out for the tool to the part at the ends of the cut. If you wanted to automate a bit more, you could use an encoder and do some software work in LinucCNC to set them by positioning the ram and pushing a button. This is another reason I like LinuxCNC, there are existing M codes for digital input and output that can be used for this or turning on an oil pump, if you don't have a mechanically driven one.
The real problem that I see with a CNC shaper is the lack of a CAM tool to build the G code files with. I've been working on something in Excel VBA, but I'm no programmer, so it has been slow going. The CAM tool should be able to make facing cuts (cut starts on the side of the work and goes to the other side or the top of the work), plunge cuts (cut starts on the top of the work and goes to the bottom or out he side), slotting cuts, dovetail cuts, inside and outside radius cuts and other specialized cuts such as gear generation. It should be able to do all these cuts at any angle the tool is capable of. That's a pretty tall order, but I think I've figured out (part of) the math and logic of it, I'm just having trouble with the programming end of it. They still make new shapers in China and India, and I checked once long ago and they sold a CNC shaper from China. Don't know if there was a software package that went with it, though.
Good luck, that's quite an ambitious project.