For a used trawler where the replacement cost of a bad engine could be 25-50% of the total boat cost, I would consider doing a separate engine survey. They are expensive, maybe $1,000 but they can be a good investment.
I have worked with several surveyors over the last few years, mostly with me on the selling side, and I never thought that they knew enough about engines to protect the buyer- not that there was anything to worry about with my boats ;-).
I just went through a survey last week. The surveyor didn't watch the temps as the boat ran at wot. He only asked me to run it at wot for maybe 30 seconds and all he cared about was wot rpms- dash rpms in fact and not phototached rpms. It takes several minutes of wot running to see if the temps are creeping up too high.
If the engine starts quickly from cold, don't smoke after warm up, runs up to rated phototached rpm in gear and don't overheat, no gas bubbles in the antifreeze overflow tank, fluid levels don't change after running, no water emulsion, valve train is clean with no rust, then you can probably forego an engine survey. But if any of these look suspicious, then do an engine survey.
David