i would guess the reason no one rates car engines like that is because no one wants to buy a car after someone says “this engine will on average last to about 250,000 miles”
manufacturers want everyone to think their cars will last forever 
but, for starters, two engines that you can put on the list of super dependable and long lasting are:
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the buick 3.8L v6
why? this engine is probably one of the longest running designs still in use today… a history of this engine here
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the lexus 1UZ-FE, which is the 4.0L V8… according to a lot of people here abroad, the replacement engines are easily bought for under $600, which is really cheap especially for a high tech dohc motor… the reason being they last forever… 300,000+ miles is no big deal to one of these engines properly maintained…
i think a good stat to look at regarding durability might be to check and see how long some older engines have been in use… most likely, (as in the case of the buick v6) the better the motor, the less reason to stop using it and replace it with something else.
the newer LS1 and LS6 engines from GM are also amazing engines that should last a good while, even when making amazing amounts of power…
gm also has some nice new engines coming out (the XV8, 12, and 16)… i should remember the inline six thats currently in the trailblazers.
oh yeah… toyota’s 3.0l supra motor… capable of handling amazing amounts of power… they usually last pretty long when not beat on too…
just some suggestions… i dunno really how you could go about doing this without a big institution backing you up, with a bunch of engines on stands, etc…