It's probably not the temp gauge. Nine times out of ten it's a bad circuit. You need to unhook the sender wire at the temp gauge (it's located in the head between cyclinder 1 and 3. With the sender wire off, turn on the ignition and check the gauge...it should read below 100 degrees. Now ground the wire with a jumper and check the gauge with the ignition on, the gauge should read way past 260 degrees. It this doesn't happen, I bet you've got a break in the plastic circuit between the oil gauge and the temp gauge (the oil gauge works, doesn't it?). These gauges share the 12 volt circuit. Voltage passes through the oil gauge then to the temp gauge.
Check the circuit with an ohm meter and also use a mangifying glass to visually check for breaks. Get rid of the alien temp gauge!
Now that I think about it, you've probably got the sender wire hooked up to the foreign gauge so the test above wouldn't work. You need to trace the green wire from the sender and undo what has been done to get the car back to how it's suppose to be.
Good luck, it's not really that hard.