My way of thinking.,,
1.- Those automotive engines are not design and will not stand 75% power output 100% of the time, as an aircraft engine do.
2.- Rotary engines have many cooling, lubrication and fuel consumption problems to be considered as airworthy.
3.- An old aircraft engine as a 320 or 360 has more than enough power and reliability to be used on the LE. Also they can easily be updated with electronic ignition and /or electronic fuel injection (http://www.flyefii.com), performing much better that any automotive engine.
4.- If you are planing to fly long legs, as I love to do, please thing about reliability first and performance second.
LE s/n; 1521