Did you know there is a software FM synthesizer for Amiga?
http://aminet.net/package/mus/misc/fmsynth37Not real-time, but you can use the samples in other stuff.
Also, it might be easier just to buy an old Yamaha FB-01 for $50 and a $5 Amiga MIDI interface from eBay and then use that. There is even an FB-01 editor available on Aminet.
I know you probably want to go the hardware route, but it seems like it might be more convoluted than the above solutions.