There have been some members experiencing problems with the thermostat in earlier BMW 1602 and BMW 2002 models. They have given some thought to replacing the thermostat with the old-style drop-in unit, as in 1967 2000's or the Bavaria.
This is certainly possible, but not practical, due to the cost of the change and the fact that there is no guarantee that the other kind of thermostat will cure your overheating problems. One member said that he had gone through three thermostats at $20 each. Other than the fact that he should be purchasing the thermostats at less than $10 each from a Roundel advertiser, if the thermostat is actually defective, it should be returned. If the thermostat functions, then something else is wrong within the system.