I’m just wondering what you mean by ‘sound’ and ‘tonality’? Could you explain the difference so I can understand more?
I used to have a Mylodica (before the wood warped), and I can say that the wooden casing didn’t seem to have much effect on the sound.
I think it would be very difficult finding replacement reeds for your melodica. Often buying a set of reeds for a melodica can be the same price as buying a new complete instrument. Maybe think about buying a secondhand Yamaha or Suzuki, both of which are quite highly rated.
I’m all for experimenting though – it would be great to see some images of what you make. I imagine wood would be easier to work with than metal, and could look quite nice.