What price frame are you thinking of?
1 yes and no, yes i can, no i would rather do 3 actual retrofits than do circuit design.
2 I have one attached to my retrofit log, not the final one so no idea what is missing or how did i loose the final one!
3 This is where the price comes in, they use Honda connectors, not sure if they can be found or at what price, but pretty sure not cheap, although there are 4 of them, so even at 50$ a piece still not expensive.
4 Mesa 7i97 should do just fine if it has 32 inputs and 16 outputs, if it has less a 7i84 will suffice, there are not much IO there as the tool changer has it's own board.
This also leaves the PC at the console and only two cables going there instead of a bunch of them, one for power the other STP/RJ45, simplifies things a lot.
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Making Honda to Mesa connectors, Mesa to tool changer, and the e-stop, enable, etc.
BTW the original drives output 0-3V depending on motor load, so adding an arduino/ESP32 with good 12 bit ADC is preferable since LinuxCNC can adjust feed depending on load, would save a lot of tools.