If you open the Panel.cfg file (with Notepad) you will be able to change the gauges.
In FS2004
If you wanted to change the gauges in the Cessna C172 you would need to locate the panel.cfg file in
C:\Program Files\Microsoft Games\Flight Simulator 9\Aircraft\c172\Panel
Open the Panel.cfg file with Notepad and you'll find entries like this:
[Window00]
file=172-Wide.bmp
size_mm=1680,1050
position=7
visible=1
no_luminous=1
ident=MAIN_PANEL
gauge01=Cessna172!Airspeed, 267,468,185,185
gauge03=Cessna!Attitude, 453,468,185,185
gauge02=Cessna!Altimeter, 643,468,185,185
gauge17=Cessna!VOR1, 839,468,184,184
For example this draws the Cessna172 Airspeed indicator 276 pixels along, 468 pixels down and makes it 185 pixels wide and 185 pixels high
If you wanted to replace it you'd first have to identify the gauge (from another aircraft) and look up its 'code' from its respective Panel. cfg file
Lets say you wanted the Beech Baron Airspeed indicator
So you'd cahange the code from
gauge01=
Cessna172!Airspeed,
267,468,185,185
to
gauge01=
Beech_Baron!Airspeed Indicator,
267,468,185,185
By changing the numbers you can alter the position and size.
Hope this gets you started
Regards
Jim