At some point, getting a certification for the auto V8 derivative becomes cost-competitive with the other options like high dollar turbines. A few companies (Orenda and others) have tried and made some progress, but I do not know exactly how far through the program they are. There's a rumor that Chevy thought about it, and their liability lawyers killed it.

Some financially-gifted wizard could come up with the cost-benefit analysis... an engine whose basic core can be bought for $5000, and produces 400 reliable horsepower on readily available cheaper car gas, but that you have to spend X dollars to certify, versus an already certified engine that costs Y dollars to buy, and Z dollars to overhaul, etc.

There is some point where the expense of the new certification is justified. Whether that point is 100 engines sold, or 10,000 engines sold, I cannot say.

There is another potential option worth considering for your super bushplane idea.

I believe the 350HP Jacobs radial engine is still being produced. Someone has even developed a fuel injection system that makes the fuel burn much better. Imagine a half-sized C-119 "Flying Boxcar" type airplane with two of these radials, de-rated to 300 HP each, and equipped with the fuel-saving and reliability improving fuel injection system. In fact, Russian airplane builder Antonov years ago built a little twin engine utility airplane for their "Ivchenko" radial engine.

By using a "Boxcar" layout, or the "Short Skyvan" type layout, you would have better prop clearance, lower loading deck height, shorter landing gear, twin engine safety (if you believe in it), and a lot of extra room inside the airplane for the same overall footprint. All you'd need is a STOL wing sized for the purpose and whatever size tires you need for your requirements.

I have no idea what a new Jacobs engine costs, but it's a HELL of a lot less than a turbine.

Not cheap, but certified.

http://www.traceengines.com/