Haha ! Very very fun project !
Long time I want to build such thing... But never found the time to !
If you are interesting into YMs fun projects, here is a site with similars implementations for many yamaha FM synth (YM2151 included) : http://sue.niko.to/ps98/
Schematics are for hooking with a Cypress EZ-USB FX2, but can easily be adapted to arduino.