There is nothing, and I mean NOTHING that has come out of the auto industry that can compete with the TCM or Lycoming engines at the power settings we consider "normal." According to one engine test engineer in Detroit, if the best engine test facilities in Detroit run their best auto engine at 75-80% power, they can only get about 300 hours out of them. If it were easy, everyone would finish the 24 hours of LeMans. Not all even make it 24 hours at the same powers we ask of our aircraft engines every flight.
I don't think that's really true. Auto engines would be getting well over 1 hp / cubic inch - more than 2 hp in many cases. Aircraft engines get around 0.5 hp? So 80% power on an auto engine might be 200-300% of the power you get from the same capacity aircraft engine. How long could an IO-360 sustain 400hp? I don't know how much power they get out of LeMans engines. I wouldn't be surprised if it was 3x what they get from a production engine, so maybe 10x the power per cubic inch of a Lycoming?
Maybe power/capacity is not a fair comparison - but it's better than a percentage of an arbitrary number that was set in part based on the reliability required.
To be frank, auto manufacturers can't afford to make engines as fragile and finicky as aircraft engines or their reputation would be junk. I have no doubt that if you had a projected market of a few million engines, auto engine designers could easily design and build an engine that was smaller, lighter, cheaper, more powerful and more reliable than current aircraft engines.
The problem is then the aircraft manufacturers would say "What we really need is something about the same size and the same weight as the current engines. And not too much more power either..." (or you run into potential problems with VNE etc) So you need to design new aircraft too. Unfortunately, the market doesn't exist to support it.