The engines are as good as the owner wants them to be. Follow the maintenance schedules and the engine will last as long as the economical life of the mower. When the major component repairs begin on the mower the engine should still be performing well.
Just changing the oil and filter isn't enough. The rpm's need to be checked every few months and the overall cleanliness of the engine and especially the cooling system are very important.
Another component of long engine life is proper maintenance and operation of the mower. It ain't a bush hog, it's a lawn mower and the blades should be kept very sharp to decrease the work load on the engine.
Oldtimer