Here are a couple general ways to decrease running temps. Since it is an old bike, definitely flush the radiator with vinegar to clean debris and improve the contact of the metal to the coolant. Use a higher percentage water mixture in the coolant - since you're in a warm country there isn't much risk of freezing. Use 'water wetter' instead of ethylene glycol. Get your hands on a lower temperature thermostat. Improve the sealing and ducting around the fan so there is more air flow when the fan is on. Use a lower temp fan switch.