The neutral gear sensor is inside the engine, if you follow the wiring coming out of the engine (just above the sprocket) you will find a singel blue wire with a connector, this is the the one... I you open the connector and hook up a volt meter between the end that goes into the engine and a grounding point you can measure if the switch is working. The circuit is closed (12V) when in neutral. If the light still does not come on it may be a faulty bulb in the dash or a broken wire.
You will find some wiring info here
http://www.mzskorpion.net/page10.htmAlso it is a good idea to download a SZR or XTZ manual for troubleshooting electrics etc, thay are essentially the same as for the MuZ, test procedures for thermal switches should be in there.
Best place for digital speedsensor is probably front disc.
As for the rev counter, waht does the instructions say? I'm guessing that you should connect it to the (+) imput terminal on the ignition coil or just wrap it 2-5 turns aroud the spark plug lead to pick up the pulse.