Maybe a quick fix would be to put a small temperature controller on it and enable it with a signal from the PLC for example a spare contact off the safety relay or another relay that is energised when system is ready, however, there could be an interlock in the PLC that stops operation until unit is up to temperature but this would require an analogue input to the PLC, this could be overcome be it analogue feedback (temp sensor with analogue output to feed back) or if a digital one a contact off the temperature controller is up to temperature.
This might be a cheaper option.
I would be careful about dismantling the PLC just in case this would disconnect any battery used for backup, not familiar with the 1200 range it could be a battery less unit, in which case there should be no problem.
However if you decide to bite the bullet & buy a programmed unit from the supplier then you could try to repair the other, try & get the code uploaded & keep it as a spare.