It all depends what you mean by ‘scratch built’. Marc (editor of GR Mag) recently posted in an editorial that he didn’t think making parts using a 3D printer was ‘modelling’ or something along those lines.
Personally, I think that if you convert and modify a drawing and have the item printed in 3D, then is it scratch built. Just because you used a printer tool instead of a carving knife it is still your creation.
However, ‘kit bashing’, which a lot of us practice, may or may not be ‘scratch building’. Bruce’s steam engine is a good example - there’s a complete but different 2-8-2 locomotive buried under all those details. His little diesel is indeed scratch built - we saw the pictures.
When does modifying an existing item become scratch building? If you make a whole new car but use off-the-shelf trucks, is it scratch built?
Or an even more complex example - EBT caboose #26, which I modelled a couple of years ago, was prototypically built on a flat car. But I didn’t build the flat car - I did what they did and made the body on an existing flat. The body was a laser ‘kit’ put together by Old Iron Designs and Great American Locomotion - I had to supply doors and windos from Ozark and a lot of interior details, wire for handrails, etc. So it is a unique model that I had to put together, and it wasn’t a kit bash, but I wouldn’t call it scratch built!