Re: Speedometer sensor broken - buy generic sensor?
The "sensor" you are referring to is a "pitot" pickup, very similar to ones used on an airplane. You can get one at just about any marine dealer, heck I think I got my last one at Fleet Farm. It's not so much that they get damaged as they get plugged up, if yours is plugged, disconnect the line going to it and blow out the pickup with an airhose.
If you want to make sure that the line doesn't have a blockage or a leak, just have someone blow into the line while you watch the speedometer, if you get nothing for movement on the needle you might have an issue with more than the pickup.
FYI the pickups are rather inexpensive (under $10)