One you need to get a fuel gauge needle, two it won't fit directly onto the Autometer due to the OE gauges using a larger pin and back lighting. Also note that your going to have to distort the alloy bezel, its not a case of pulling it apart you need to run round the bezel with the flat bladed screw driver / mini chisel to get the face glass to come out.
Re-sealing isnt a case of using sealant its a case of trying to get the metal you've just creased / stretched to re-fit, this is why Autometer fit a new bezel each time they open up a gauge as its extreamly hard to get the old one looking good.
Anything is possibly, which your at it you may aswell make a new face and numbers that back light like the Cupra as your already going through enough hastle opening up the gauges.
Other options would be some Stewart Warner gauges or Autogauge (they do one with black lit numbers, its a 7 colour gauge).