Please don't misunderstand me, but I don't think your thermostat is impacted by tropical weather. It doesn't know if it's at sea level, on a snowy mountain top, in a desert, or in the middle of a tropical rain forest. It only knows what temperature it's supposed to open at as the radiator coolant heats up. Thermostats come in various temperature range ratings. As was suggested, go to your local Nissan dealer and get a new thermostat. Don't ask for a tropical one or an arctic one. They will have the one that will work just dandy in your car. You could also go to your local Ford dealer as they also have one that fits the Z.
Is there another mechanic in your area who can work on your Z? Your current mechanic made some real career limiting statements and should be avoided in the future. Assuming there are no other mechanics who will work on your Z, you have two choices. You can work on it yourself with guidance from members of this forum, or you can sell the car and get something a little more mainstream for Panama, like a Miata or a BMW.
Are you the only Z owner in Panama? Find another owner or a local Nissan/Datsun Club to join. You'll get good advice on local repair specialists. With all the problems you've had with this car, I'm surprised you still own it. There comes a time to either get it fixed properly or move on to a better car. The car should provide pleasure, not constant headaches. So, unless you get pleasure from headaches, you may want to consider a more reliable, less maintenance intensive, car.
Let me know what you think.
Dennis