It doesn't surprise me in the least that circularity on the casting would be messed up, you would just think that the person on the mill would have the presence of mind to take a look at it before sending it off to the end-user. We reject castings pretty regularly, and that ID could be turned true without a great deal of difficulty provided the deviation in roundness didn't exceed something like 1/2 material thickness.
What doesn't sit right with me is the fact that for a small run, the price of even an aluminum casting would absolutely motivate me to get it right the first time. At the level of volume that the guy would have to be operating at, I would be test-fitting every part to a car before sending it out the door.
I feel like the relative scarcity of air-cooled 911s coupled with the difficulty of machining the part correctly as a third party is why the part itself is so hard to come by. If a person were motivated, though, there is definitely another way to make a suitable replacement more cheaply and easily with a modified design. That's provided someone is okay with not having a completely factory 911.