Look up "Unity Gain Op-Amp" and then see if it would be worth the effort. Cost you $4-$5 at radio shack you do need to look at data sheets to make sure you build a stable circuit but is not that hard... but then again I am an electrical engineer....
Personally it seams like you just want some flashy lights. a $30 A/F gauge is going to tell you nothing and most likely is not very accurate to start with. And what has been said above it will stay fixed at basically 14.7.. may drop to 14.6 or 14.5 at the high revs. So it will always stay ideal. I can wire you up a dial with an LED you plug in that will always illuminate lettering that says "Ideal" :-) give you good price....
What other gauges do you get a good discount on? go for oil pressure or oil temp or volt? those 3 you can easily do and will actually provide you with good information and be easy to install. You could also do a water temp, the gauge one is not very accurate. Its use is limited to Nominal temp and better shut the engine off.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)