It is just weight that's the problem? If so, what muscles would need to be exercised to allow this to happen?

For function, weight and flexibility are important. It's not just a case of increasing muscle to make falpping heavier wings possible though,  as bigger muscles add more weight. More weight (total, not just wings) means bigger wingspans would be needed to provide sufficient lift. This is turn could limit manouvreability or access to habitats (you need small wingspans to operate in a forest). External "armour" might also interfere with other functions of feathers (insulation, waterproofing etc) if it were applied to extant birds.