The Westfalia did not have a bathroom fan when imported. Some owners did retrofit a Fantastic Fan instead of the bathroom ceiling vent. Key is to use a European size frame as the Westfalia has a European size opening of 16" x 16" or so, versus American standard openingof 14" x 14". (Note: There are some European brand ceiling fans but am not aware of anyone in USA who used those.)
If you can confirm you have a Fantastic Fan, you may be able to just replace parts or most of the Fantastic Fan, but keep the Euro sized frame. The onky difference is the frame--all other Fantastic Fan parts should be the same for the given model.
See this website where many owners have purchased the Eurosized frame, and fan:
http://www.rvvent.com/
May want to contact them with info on what type of fan you have (look at remote control, if have one, and email photo of fan to them if needed), what options you have to repair, replace most stuff, or whole replacement. There are various models but for the effort involved, might go for top-of-the-line model.
If don't want to do yourself and if a Fantastic Fan, any competent RV repair shop should be familar with Fantastic Fan.
Also, search this subforum and the Yahoo Sprinter Westfalia group for "Fantastic Fan" for more tips.
Please post whatever you do. Thanks.