The most effective and accurate way to tell if an engine needs a rebuild is a compression test. Hour meters are good for preventive maintenance, but actual wear and tear can happen at differing rates to identical machines doing identical service, even if they are running the same fuel mix.
Compression tests and overall cleanliness/condition of the engine, engine space and entire machine will tell you better than an hours meter.
JMHO, YMMV. I was a small engine mechanic for 20 odd years.
LX in #2 has a better detailed reply than I. He’s quite correct in what is said there.
Engines looked leak free. Exhaust systems were sound. wiring good. *IF* the bilge had been pressure-washed, you’d be hard-pressed to know these were 12 year-old motors.
Never done a compression test on a small displacement 2-stroke; only 4-stroke automotive - mostly 4 cyl.
Is it as simple as removing one plug, cranking the engine for 5 seconds and timing the leak down? (then repeating fore other cylinders)
I don’t know the proper leak curve ;-)