I lean towards replacing the in-game BIKs completely rather than merely extending them. As you pointed out, it enables to avoid the issues of assets you don't have access to and using any sound design and music that you wish to use. Seeing as you would be making a higher resolution (and larger) movie, not reusing any of the original materials to artificially extend the preexisting video is actually a bonus.
I would cheer for new versions that make use of your current talents and the latest available tools.