Finally I realized that LMMS has some sounds of timbales and drums, which must be free since LMMS is free. I will try to reduce their quality, convert them into dcm format, and then add them to the FamiTracker file to replace the previous recorded samples. That should work.
Anyway, a composer that makes SNES-like music has told me that it's highly unlikely that I would run into any legal problems with those tracks, that most of the samples used in SNES games were actually down-sampled freely usable libraries, and that many people have used samples from real SNES games in commercially released soundtracks. Who knows if this is really true...
But in any case, since I am quite paranoid, I am gonna take the slow but safe and legal way of replacing all recorded samples.