Nice song - I enjoyed listening!

Some years ago, I had a number of songs my church choir had recorded, and I decided to embellish them with BiaB. There are two approaches, and they're both tedious.
The key is
time stretching the audio to make it fit. You either stretch the existing audio to fit the BiaB track so it's on exact tempo, or you can stretch the backing tracks so they match the tempo of the original recording.
Depending on the amount of tempo drift, this could involve stretching at the measure level, or even at the beat level.
In Reaper, the are
Stretch Markers that make this really easy to snap the audio to a grid. The other route is doable, but a bit more difficult, as you have to align the markers to the original audio instead of the grid.
Again, it's tedious, but it's certainly doable.