What about the other fan? The one on the actual motherboard... does that one have a brass sleeve too? It's smaller I believe.
And yes, I believe the case fans are the same as the CPS2 fans - 60x15. The airflow however may be different, I haven't looked at the CPS2 one in quite some time.
Edit: After doing a bit more research I think I'm damned if I do and damned if I don't. A replacement fan (other than the Papst) is almost assuredly going to be louder in order to justify the airflow, and the Papst fans that I've found are all going at about $15 bucks a piece. Maybe I'm better off just attaching the Naomi board to the cabinet with some rubber washers or something else that will help dampen the sound. (I mean this in the sense that the fan that's in there is probably operating at it's normal volume)
Edit #2: Just for the fun of it, I took CM's advice and cleaned it out the way he suggested. I applied one drop of lube, and while it seems a bit better, it's still making the same amount of noise - so the fan appears to have been fine all along. Of course, now it will last even longer.
Repairing these particular case fan's proved rather difficult, as instead of having a rubber cap in the middle or nothing at all under the sticker, there was a type of recessed plastic. But once you get this off, it's cake from there.