A couple things,
Car engines will work with minor modification in small ducted fan planes.
Car engines tend to be way over-cooled so cutting off a bunch of cooling head fins would be in order.
I have seen a couple car engies that had enough crankshaft sticking out to where a thin prop could be used, of course a prop collar would need to be found.
If you can get a car engine up into its torqueband with a prop, I would not want to be anywhere near a prop spinning 30K (read ; very very dangerous, even when absolutly perfectly ballanced).
As soon as you increase the load on a car engine (ie;using props not gears), mor oil will be needed to protect the rod and crank pin area, which lowers the performance (but increases the low end torque) and makes it harder to get on the pipe.

Can it be done-- Yes
Have I done it--Yes
Is it worth it --NO
Is it practical-- NO

Just my experiences