Just about any Modern Nema 34 motor will beat those old round steppers so wouldn't worry too much.
Also I would consider replacing the drives with Modern Digital drives. Lead shine EM806 or AM882 drives are excellent drives and pair nicely with smaller Nema34 motors. There advanced resonance capabiltys are great for Mills which tend to be very stiff. It's Night and day difference in how the perform compared to old analog technology.

If you do want to go with Closed loop then On Ali express there are some good deals.
Also Consider Servos if buying from China because they don't cost much more money than closed loop.

If wanting from UK then you'll pay lot more money for closed loop or Servos.