Congrats on finding a Dolch PAC 65! Much envy...
I investigated making a non-invasive adapter for my Dolch PAC but when it arrived, I was impatient and just soldered a salvaged PS/2 cable directly to the pins on the PCB
Since the stock cable has a plug on the inside, the mod is reversible.
I doubt there's an off-the-shelf adapter you can buy - the connector pinout is totally proprietary on top of rare and vintage. You can make one yourself pretty easily, though. Assuming it is RJ45 like that link you posted, get an RJ45 breakout PCB and socket, e.g.
https://www.sparkfun.com/products/716https://www.sparkfun.com/products/643... and then solder PS/2 cables to that, or wires from a Teensy (or something similar) for USB conversion.
If you want to get the trackpoint working as well, and don't want to deal with two PS/2 cables and two USB converters, it's theoretically possible to get a Teensy or something to convert both PS/2 keyboard and PS/2 mouse via a single USB, but it may require building a custom firmware, maybe Soarer's or TMK can do this without too much hacking.