Unlike cars, even old aviation engines have strict rules for inspection, and for replacement of parts after a certain number of hours, rather than when they wear out or fail. You could get away with circumventing these for a while, but it is simply not true that it is "very common" for "old ass prop planes" to have engine failures, comparably to old cars.
> It was also strongly speculated that he filled one of the fuel tanks with water...
I would be interested in seeing how strong the evidence behind this speculation is, but if it were the case, it would be yet further evidence of his incompetence, as water is no help, and sometimes a problem, in preventing, containing and extinguishing a gasoline fire.
Even in an empty fuel tank the vapors are way to rich to ignite. You might get a good flash if you introduce air and ignition at the same time but odds are you won't get any sort of sustained fire.
For instance, the apparent reason why the engine stopped working isn't due to mechanical failure of the engine (very common in old ass prop planes, just as your Hyundai might blow up, due to oil starvation, spun bearings, rods grenading itself, etc), but rather running out of fuel. He purposefully brought not enough fuel to kill the engine, and secondarily prevent a fire. It was also strongly speculated that he filled one of the fuel tanks with water, probably in an attempt to prevent a fire from erupting.