You are right GBeer that this part comes from a Gowland camera. Unfortunately Peter Gowland passed, so getting parts is not quite so easy anymore- if indeed there are any parts left. My part isn't cracked- it's just a bit deformed inward in spots making focusing difficult.
Thanks for the responses everyone- I will probably end up getting a machinist to do this if I can't source a part. It is encouraging to know I could do it myself if I just had a tablesaw though..
You might ask about having the part made from alternate materials. 7075-t6 aluminum in much stronger/harder than 6061-t6. Less likely to deform. Or you could opt to have the part made from stainless steel. Be sure to show the machinist the camera and tell him of your need for smooth but tight operation of the focus.
Now that I know the problem is with smooth focus. I'll say definitely, go to a machine shop. Trying to get the necessary tight clearances and finishes just won't happen with a make do operation.