Two things that indicate an engine wearing out.

1. Low compression in ALL cylinders (one cylinder only means valves).
2. Low oil pressure (bearings are wearing out).

Every time the spark plugs are removed you should test the compression.

Every time the oil is changed you should inspect the drained oil for aluminum bearing particles.

The best way to check for worn bearings is to buy an oil filter cutter. If you cut open every oil filter and carefully inspect the filter element for particles, you will know when the bearings are going bad (and the distributor gear).

Dirty job but one of the advantages of changing your own oil!
Dennis Moore/Moore Performance

Very good post Dennis Moore. Exactly what boat owners should do to check their engines for proper steady state use and conditions.
As for Mag versu HO engines their is absolutly no internal differences mechanically or operationaly that will give different expected engine life. Same engines just different camshaft profile and ECM programming differences do to camshaft differences.

Best Regards,
Ray @ Raylar