Bonjour Gil,
There is certainly a similarity between these two engines but also a big difference, mine has no exhaust! It seems that this is caused by a small leak intentionally created at the level of the cylinder head because I unscrewed it and saw a small notch, even weirder...

I was told it could be a Scudery engine :
http://ridders.nu/Webpaginas/pagina_scuderi_viertakt/scuderi_frameset.htm