The largest transformer most likely steps down the AC line voltage which is then changed to DC with a square wave rectifier then smoothed with the large cap. This would supply power to the servo drivers.
To get the spindle running, the easiest way would be to get a modern VFD of the proper HP rating to drive the 3-phase motor you show. The Acorn's 0-10V analog spindle output will drive the VFD/motor to the desired speed.
You might want to think about moving up to a Centroid Oak because Acorns only output step & direction signals & won't control the analog servos & drives that mill has. Acorn will work only if you change out at least the drives & encoders and most likely the servo motors as well. ($$$)
I suggest you go here: https://centroidcncforum.com/viewforum.php?f=60 and ask your questions. There are many more knowledgeable folks there than me that will most likely help you with the best solution. It's gonna be a big, expensive job but well worth it if the basic mechanical parts of the machine are in decent shape.
Good luck & post lots of pics!