Here's something to ponder.
When we build our favorite model. Do we really know the subject? Do we truly know when this or that was added or deleted? I wondered about that because of a question just answered in the forums by me. The old Warships of W.W.1 and early W.W.2 were not the super fast, super electronic bag of tricks they are now. Nor as Blah! Looking!
The same can be said about the rest of anything Military or Civilian, in models. Now that same philosophy can be seen in other areas of model building. In Trains, Very few of the Building Kits reflect modern cityscapes, yet they aint cheap either!
Model Boats have a few " Pods" as I call Modern Yachts. No Real standard Pre Sixties Yachts of note are offered in plastic at all. Now here's where the Title for this comes in. Not necessarily Scratch-Building. But a good group of transitional models of all types can be made. How? By doing what the title suggests.
There many things out there in other realms of What is called Crafting or Hobby pursuits. Model Building is but a small part of what's out there. Many, if not all, have parts and pieces that are interchangeable. Most Diorama builders know this.
So decide which Era and Genre you are going to build .Then, go out to National Guard Units and Air Bases as well as Navy Bases ( You probably can't take photos now) And get an eyeful. Then go home an take what you've seen and apply it to your models. If you don't have a part then take your parts BOX and come up with a Greeblie that works. In other words. THINK OUTSIDE THE BOX ! ! Note: Be as observant as you can. You might just see something at the periphery of your vision that is just what you need!