how to edit the text shown on the end screen
It's a bit tricky, since you can't use Lunar Magic to do so. You have to use YY-CHR to edit the GFX file manually (filename GFX2F.bin, I believe) and, even then, the way the graphics are organized makes it a bit confusing.
Personally, I'd suggest leaving that file unedited. But you're free to do as you wish.