The size you gave looked familiar. I just checked, the Version one and Version 2 Mikado (gearbox bolts down to the frame) is a 6303 at both ends.
I have three of these engines, restored. not sure about parts. I can dig through my stuff, but a broken Version 2 Mike is probably a lot easier to find for that part!
If I recall, there is a slot, depression, divot on one side of the outer ball that fits against a ridge to keep it from spinning.
Dave
