Okay.......
Not all 911 fans are the same.
Is it an original assembly, from a Porsche, or some aftermarket repop made to be sold for a profit??
You can tell easily by the 901/911/930 prefixed part number on the fan ring, or if you have a pic I can tell by just looking at it.
If it is an original, you will have no problem finding the alternator, but you MUST get the same part number as the old one. There were 4 different amperages of alternator on a 911 35,55,75 and 90 were the sizes. each alternator and ring assembly MUST MATCH EACH OTHER or you WILL have problems with even making it work.
If the alternator/fan assembly came from some aftermarket company, the chances are you will have to buy an alternator from some year of VW Golf, and make the mods to make it fit...
Good luck.