What specific tests did the tech perform on this powerhead?

There is a diagnostic procedure for most engines I have worked on and
they usually pinpoint the cause of the problem. This starts with a
basic visual inspection, then moves on down the line to a simple spark,
compression, fuel and other simple tests. If any problems are detected,
then often a teardown of certain parts of the powerhead is required in
order to have a closer inspection.

It almost sounds like this tech didn't do his basic checks. I would
recommend getting a service manual for that powerhead and having a
closer look at other possible causes. And maybe find a new tech. Just
my opinion.