The signal at pins 1 shows the LT1013 is correctly calling for a lot of heater power, so the temperature measurement may be still working, at least it is not the reason the heater is off.
One could check if the temperature-sensor is still working by applying a little more current to J400.1 (e.g some 47 K to 7 V or 15 V). This should shift the set point to a much lower temperature, so that the OP should turn off the heater. If this still works as expected, the temperature sensor seems to be working, which would be a good thing.
Chances are one could than add an external heater and still use the internal sensor. It would very likely need a modified control network (e.g. considerably larger cap for C412) as there will be more delay.
The voltage at pin J401.4 should be negative, for a standalone test one can also just keep this pin open.
One should check the voltage at pins 1 and 2 of the LTZ - there is a slim chance the bad part is the diode or transistor of a bad solder joint (unlikely, but not impossible).