The kit doesn't include these parts?
One of the challenges associated with scratch building (or kit-bashing) is that you need to purchase dimensional strips and sheets if you don't have the tooling (or inclination) to process wood yourself:
Precision hardwood for ship modellers, luthiers, CNC and laser work: Swiss pear, Castello boxwood, black hornbeam, Alaskan yellow cedar and maple — planking…
hobbymill.eu
The problem is you will need to purchase lots of dimensional stock because you won't necessarily know up front what you will need. Of course if you had or made a thicknesser (thickness sander) that gives you more flexibility.
I suppose these particular parts
could be made from basswood - but that would be pretty much my last choice