I suspect that the gray motor is exactly that - a three phase motor being used to generate three phase power from the standard single phase 220 volt line being started manually. If you add a static phase converter to the gray motor (connected to the motor directly) you will have created a self starting, rotary phase converter. Just apply power and away it goes. Check this ebay listing out for a static phase converter that is less than $50 with free shipping. In the listing, go to the bottom and click the "ask a question" link and then click "other" for a list of FAQ's. He talks about using a static converter and an idler motor to build a rotary phase converter. For under $50 you can use the motor you have and add the static converter to end up with a nice rotary converter that will give you the full power output of the motor on the mill and should not harm the mill motor, based on what I have read.
http://www.ebay.com/itm/3-5-Hp-Stat...864?pt=LH_DefaultDomain_0&hash=item2c7d4f8aa8
I would suggest contacting the seller for sizing information. This one is for a 3-5 hp motor - might not need that size. I saw the same seller had a smaller version for a little less.