another poor man's car engine conversion

Yesterday I was talking to a friend who plans to power his single-seat
slow flyer with an engine from a Citroen Visa. I suppose this engine (a
linear descendant from the famous Citroen 2CV) is not well known in the
US, it is an air-cooled 2-cylinder boxer, in this particular application
it would produce some 45 HP.

My friend absolotely wants it in the plane as it is in the car, i.e.
with the clutch side rearward (the plane is a traditional "puller"), and
wants to take power from the clutch side. His idea is to have a belt
reduction "behind" the engine, then a transmission axle above the engine
to drive the prop. Now I'm sceptical 'cause I heard all kind of bad
things about transmission axles driving propellers, vibration not the
least. But he answers the axle needn't be long, as the engine is only a
2-cylinder.

Any thoughts / ideas / comments / experiences?
TIA,