You're right, it was a cast part that had been machined, and (from memory) the machining was not precise enough to locate the alternator (and thus the fan) in the exact center. It kind of seemed liked a backyard job, but the price was anything but.
I think from memory the inside of the circle (which was cast) was not perfectly round. From memory the original Porsche part was titanium, and this replacement was aluminum. But I might have that backwards.
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.
A 5 axis mill? Once you had the model you could get one offs in many contract shops. It is by no means trivial but if it is an expensive part you could make a very workable copy. Given the required characteristics of the air flow there are some programs that calculate blade profiles.
Maybe the part would end up even more expensive but that sounds like an interesting and worthwhile project.
I think from memory the inside of the circle (which was cast) was not perfectly round. From memory the original Porsche part was titanium, and this replacement was aluminum. But I might have that backwards.