You understand the difference between 68k and PowerPC (PPC) code, right? The beige G3 is built around a PowerPC CPU, and the Classics/Pluses etc are all 68k.

Programs will run on one or the other, but not both. Exceptions being:

• 68k programs will run on PPC machines in a 68k emulator built into the Mac operating system. This doesn't always work smoothly, especially with very early programs.
• Programs written around the time of the changeover (mid-90s) which might be distributed as "fat binaries", containing code for both platforms.

By the way, that's a really nice collection of older hardware you have there. I use a TR-707 myself, using the individual outs to trigger my Simmons SDS-8 analog drum synth, and a Roland PMA-5 for sampled voices and patterns. (Amongst other gear)

D20 = Roland keyboard? That, IIRC, should also have a "To host" or "Mac/PC" MiniDIN-8 connector. If that is the keyboard your son is using, he should be able to connect it directly to the Mac.

Mac serial MIDI adapters turn up in the Trading Post forum here from time to time. One just changed hands recently.

You might also be interested in the "Deep Synthesis" site, which hosts information on old synths, and patches for SoundDiver to unlock hidden or advanced programming methods:

Yamaha TG-100/CBX-T3

Roland D-10/D-110 series (inc. D-20)

Roland MT-32