Repair manual is not quite as handy as researching posts on MA or looking in the Wiki. There are some clever folk who can help with all of your snags and point you in the right direction.
Seems that old Samsonite luggage key blanks work in Motobecane steering locks if you are patient enough to pick the one installed. Otherwise you will need to drill out the old and install the new. The steering lock for the serie 50 Mobylette can be purchased from MopedLand UK for £10, includes 2 keys or get the blankplug for £3 from the same folks.