You can't replace gauges in CFS3 since they are baked into the model.
All you can do is repaint them.
The not working asi is due to a strange bug in CFS3 that happens when the aircraft exceeds a certain number of virtual cockpit stations.
Apparently Shockwave choose to include all B-17 gunstations at the cost of a not working airspeed indicator.

Not a straight forward task, particularly since the FP B-17's bombardier station is the "view=9", the offending bit if you will.
First of all, backup the entire aircraft folder
Open the xdp in notepad or any other text editor,
scroll down until you see the last of the gunner station entries and remove it:

<Seat Name="Tail Gunner">
        <Stations>
           <Station Name="TAIL GUN" Type="gunner_station" View="8" FovUp="25" FovDown="30">
              <Views>
                 <View Event="HeadStop"/>
                 <View Event="HeadPitchUp" Azimuth="0" Elevation="-30"/>
                 <View Event="HeadPitchDown" Azimuth="0" Elevation="30"/>
                 <View Event="HeadPitchUpLeft" Azimuth="-45" Elevation="5"/>
                 <View Event="HeadPitchUpRight" Azimuth="45" Elevation="5"/>
                 <View Event="HeadRotLeft" Azimuth="-90" Elevation="-5"/>
                 <View Event="HeadRotRight" Azimuth="90" Elevation="-5"/>
                 <View Event="HeadPitchDownLeft" Azimuth="-134" Elevation="0"/>
                 <View Event="HeadPitchDownRight" Azimuth="134" Elevation="0"/>
              </Views>
              <GunStations>
                 <GunStation ID="6"/>
              </GunStations>
           </Station>
        </Stations>
     </Seat>
  </Seats>

Scroll up again to the pilot station, locate the bombardier string:

</Station>
           <Station Name="Bombardier" Type="bomber_station" View="9" MaxFOV="40"/>
        </Stations>

Replace the "view=9" with "view=8

Save, close and open the xdp in internet explorer to verify that there are no syntax errors. If you screwed the format it will show you an error message in IE.

Locate the file "B_17G_Cockpit6.M3D" and open it with Notepad.
Search for "eyepoint9" and replace it with "eyepoint8".
Save and make sure that notepad doesn't add a ".txt" extension to your model file :-9

If the above fails to work just reload your backup, LOL.

Okay, well this turned into a little headache:
There appear to be two extra stations above the limit:
9 is bombardier
10 is co-pilot
I removed 10 completely
I then removed the tail gunner, and renamed bombardier as view 8, as per your instructions.
This worked fine.
But now the problem came when I opened the B_17G_Cockpit6.M3Dfile in Notepad. I could change the eyepoint from 9 to 8. But whenever I saved and then re-opened to check it, all the forrmatting had disappeared. How do I stop that happening ?

I used wordpad, and that solved the re-formatting.
However, the re-numbering doesn't work, because I don't get the views I'm supposed to, and the airspeed gauge still doesn't work.

I bet the bug is in the exporter.
Never mind. It was worth a try, so thanks for your help.