Whoops, I didn’t read the Junto instructions properly.
However, the error seems to fit the theme.
When looking for a track to erase, I settled on ‘Lame Brain’ from Junto 322.
I figured I’d erase the MIDI viola and cello, then use the original guitar parts.
When I opened the project file I found I’d erased the guitar parts.
As a result, I went back to the recording and created a new arrangement.
It’s about a minute shorter than the original, which is probably for the better.