Thanks for the link! That site helped me a lot. I learned a bit about SoundFonts and I think I found one for a timpani. I got the sf2 file loaded into GarageBand and saved the instrument.
I drag my MIDI file into GarageBand. It imports and I remap my channel from "Church Bells" to my newly saved Timpani. The instrument is completely silent. Anything new that I record will play but the mappings don't seem to work on channels that already had something loaded into them.
-=- Boris