My 1991 Mazda MPV idles to the point that the engine is racing when it's in Neutral or Park. If I have it in drive I hardly even have to press on the gas to make it go forward. It has a very high idle and I just replaced the Radiator, clutch fan, thermostat and the water pump. I am at a loss as to what this could be.

Have your Idle Speed Control Valve checked for proper operation. It should close when the engine is at normal operating temp. All you have to do is disconnect the electrical connector when your car is at idle and the RPM should go down.

If you replaced the thermostat with one that is too cold, your computer will interpret the readings as though the engine is still cold. In other words... the computer still has the choke "on", but never gets to operating temperature. I'd check that first. Thermostats are cheap!!!

The temperature gauge reads the engine at normal operating temperature. Is it possible for the engine to still be reading as cold ?

Where can I find the Idle Speed Control valve ?

If you follow the air intake hose from the air filter box it should "T" off to a smaller hose which will go to the Idle Speed Control Valve (ISC). It should also have coolant lines 1 going to and 1 from it. Should be located in the general area around the throttle body.