As far as the CT test here is concerned everything has to be as it was originally produced, even different size wheels will mean it will fail.
The french tend to enforce the rules quite firmly when it comes to vehicles, unlike in the UK where you can get away with almost anything as long as it will pass an MOT. I remember going to Loudeac and someone turned up from the UK with a much modified R4 (possibly Mr. Renospeed), which had a five speed box, bigger engine and possibly even a turbo. Standing around the car were several french enthusiasts, the concensus was 'that would not be possible here' even if they would have liked to do it.
The particular type of 2cv chassis had no bottom plate from memory - making it easier to rustproof on a regular basis.