According to the Cady owner's manual which I got a hold of recently, the gap should be "4/10 mm (.012 -.016")."
The Cady service manual says that the following plugs will work:
Bosch 436, A C 430Z, Champion L88A.
The Cady seems to have different specs than the other motobecane models.
It's electrical system is 6V.
PS. The owner's manual also says to put in a 25:1 gas oil mixture.