I ran into a similar problem the other way around. I wanted to map OrgMaker instruments to MIDI instruments. I eventually gave up trying to make a one-size-fits-all mapping, and decided to carefully listen and choose matching instruments by trial and error each time. But while I was trying to make a mapping, here are some things I noticed and tried. Maybe my ideas would be helpful to you or anyone else trying the same thing.
I extracted the wave data from OrgMaker and graphed what the waves look like:
wave100-graphs.zip. I couldn't predict what sounds would match the wave pictures, but you can at least group similar looking wave pictures into families, then listen to how the OrgMaker instruments in a family sound different.
All ORG instruments have no decay. That is, they don't fade away. As long as OrgMaker's volume level graph isn't changed, they have the same volume for the entire length of the note. If you're converting from ORG instruments to MIDI instruments, for example, this means you probably don't want to use a MIDI instrument like a piano (when the note is long, the sound fades away), but something like organ or flute (the sound stays the same volume for the entire length of the note). When you're converting other instruments to a matching ORG instrument, this observation may have less effect on your choices. You can just use the volume graph in OrgMaker to create the decay you want.
Using an audio player with a spectrogram plug-in, I took screen shots of the spectrograms of all of the ORG instruments, and of all the MIDI instruments with no decay, all playing the same pitch. My idea was to match instruments with similar looking spectrograms, but I only found a few matches (those with only a strong fundamental) then gave up. Because of the kind of spectrograms I made, it was difficult to compare the higher harmonics. If I restarted a project like this, I'd look for a different kind of spectrum analyzer, and make notes about the harmonic strengths, instead of relying on just a visual match.
