Put the car on stands and get under the right side near the gearbox, straight under the drivers seat. It's easiet to have the car jacked up and sitting on the two left wheels and support the weight front and rear on the right side. MX73's are damn low riding cars so are a pain to work on from underneath.
There is a lever the comes down from the shifter and connects to a horizontal rod (about 50cm long)that runs towards the front of the gearbox. This bar in turn connects to the switch in question(via another short lever). The neutral start switch is mounted to the front right side of the gearbox.
You'll see that, where the bar connects to the gearstick/shifter, the connection is adjustable becuase of a slotted hole. Check and adjust here aswell were the small lever actually connects to the gearbox. There is a nut there too that might be loose.