I am going through this with my 4CV – as I am putting in a dauphine engine (the told engine is terrible and in theory “broken”). But it is totally illegal to do this and when you go deeper into the rules, you cannot modify or change anything from the original homologation.
Another guy who has a dauphine engine in his vehicle said when he worked for Renault in ’59 – ’62 they would exchange the 4CV original engine with a dauphine Ventoux engine as a matter of practise when the 4CV gave up the ghost.
So changing from a perfo to a D or even a DS is against the law.
But as I have been told “off the record” – who in France follows any of the laws, until they are caught. ๐ ๐