Somewhere, there has to be a piece of paper signed/stamped by the engineer to say that the magneto has been replaced, and all the details of the replacement magneto is logged? ie form1 or whatever? And it should then be inserted/glued into the log book, which he doesn't have?
No, the insert does not ever have to be glued into the engine logbook.
It is entirely legitimate for maintenance records to be a collection of loose sheets. You don't have to keep a bound "logbook".
Or they can be writtne on the back of a pig (the most common scenario
)