'at what point would you decide to scratch the rudder or horizontal tail surfaces instead of modifying kit parts? '
I didn't see a point in this build where I would scratch either. For me, modifying an existing part is easier.
Modifying a part that is similar and already fits a condition is less time consuming and easier to do than making a part to fit. Sometimes it takes two or three times to make a part that is visually and technically acceptable. Visually is that it has to have the shape of the part. Technically is that it has to be made to look like the part.
I could cut new horizontal stabilizers from .040 sheet that in plan look just like what I've made. But an oblique angle of viewing would show square edges. To make it be similar to, or match the part, a lot of sanding would be involved. That's the technical part.
Then there is the fit problem to solve. So the first problem to solve is one of aesthetics. The part you make has to look like the part you want to replicate. Then it has to fit into the condition for which it is made.
You can make the most beautiful award-winning part in the world that is the best thing you have ever done and you want to show it to everybody because you are so proud of it. But if it doesn't fit, you have to make it over.
The parts you mention already fit a condition and, to me, only need modification to render them similar.