a = indefinite article
many a good thing (rather formal) = many good things
can be had = VP
too much of = phrasal determiner
They can have too much of many a good thing. -- active counterpart
Passivization is a left-hand NP movement: 'many a good thing' is raised out of its direct object site and put to the preverbal subject territory.
The passivized VP is: "can be had"; 'have' is a middle verb which can undergo passivization occasionally.
many a good thing + passivized VP (= can be had) + stranded phrasal determiner --> Many a good thing can be had too much of.
Student or Learner