Although this is a bit of a late reply, I did replace my shower vent fan after the blades "disentagrated" after a noisey run. Out-of-Doors Mart in Greensboro NC sold me a new fan and while I was at it, I replaced the outside cover. Working from the outside, I removed the cover and replaced the fan (it is a one piece unit) that pressed on. I also picked up all the little pieces of the old fan blades. New cover (two screws as I recall) and it has worked like a charm for 2 years now.
1994 34' Excella 1000
2000 Excursion Diesel 4x4 with Banks Stinger Plus Power Pack and Exhaust Brake