look at motorcycle engines, they get bought new by a person who usually has no mechanial sympathy an then sold, they arnt always wearmed up b4 bein thrashed .
i own a 10 yr old honda fireblade that produces 123bhp at the wheel or 133 at the crank an has spent most of its time flat out racing my mates
you dont do 170 mph unless u know ur bike is 100%.
even dealers say they will take a blade with 80000+ miles.
in the factory they test them on a bench bangin them off thier limiter at 14000rpm till they blow an that usually takes months or years providing they are serviced.
any engines is almost capable of running forever if A) its serviced regularly B) wear happens when it starts up an is cold, when its warm it doesnt really wear.
engines usually fail for lack of servicin or a design fault or a bit of swarf from the machining/casting process blocks an oil way or something terrible.
also buy the best oil like fully synthetic if ur engine is okay with it on a 4 stroke as this stuff is amazing an doesnt allow wear in ur engine, otherwise on a 2 stroke i would use £10 a litre 2 stroke oil again fully synthetic.
i personally dont see it bein hard for a bike manufacturer or a car manufacturer like subaru makin an inboard an modyfing it to run salt water.
i dont think reliablity is a problem an people who have any honda willl rant an rave build quality an reliability.