i think this problem is in every cultus,, they dont show more than half deflection when overheating,,
mistake is urs,, efi cultus has overheating sensor n it turns of engine when overheating,, u should have cheked it in very first turn off
i dont think engine needs to be overhauled if there is no oi reduction or smoke,, remove engine oil cap while car is hot,, if black smoke emits then engine should be completely overhauled
otherwise,, only change gaskit, valves and seals,,, wont cost more than 10k if best pars used