I had the same problem with the gas gauge and just replaced my gauge cluster with a used one bought off ebay. No further problem.
Your temperature sensor could need replacement in order for the temp gauge to read correctly. Make sure also that your cooling system is correctly bled to get the air out and filled with coolant so the temp sensor is submerged in coolant.
If you have pulled engines before and changed clutches, you can do it on the 944. You can buy a conventional clutch disc and pressure plate to reinstall if you want. Check the aftermarket suppliers like 944OnLine, etc. I think Sachs makes one, probably others.