Things is a car only requires what 30-40(?)HP to run a 50 MPH which is about 20% of their maximum.
Airplane engines run at leats at 65% of their maximum.
That Citroen engine would be amazing if it would run 3500hrs at an average of 100MPH, but it won't.
If this the aircraft referred to
DIYer Builds Peugeot AX 106 Diesel Engine Powered Wooden Aircraft - Auto Motto with the 1.4 or 1.5 litre Citroen TUD engine used in the Citroen AX, Saxo and Peugeot 106, it is worth bearing in mind that the French have or had a speed limit of 140km/h (87mph) on their autoroutes. The engine will have been designed to cruise all day at that speed, and the total horsepower of the engine is only 58bhp (53bhp for the 1.4). This engine is designed to operate at far closer to an aircraft engine regime than a larger more powerful one would.
I owned a Citroen AX 1.4 diesel, and my memory of the weekly 100 mile journey to Filton, along the M4, was having my foot hard to the floor for most of the way. The car already had 150,000 miles on it when I bought it but never gave any trouble.
You may not get 3500 hours from the Citroen engine, but you could probably throw the engine away, buy another and still have change from the price of a Lycoming.
Here's the original article in French:
Gaz'aile 2