you can use any mac os 9 compatible USB midi device or compatible usb midi keyboard
instantly coming to mind:
-Motu Fastlane usb midi interface
-Edirol UM-x series midi usb midi cables
-Edirol Um-550/880 interfaces
-AMT8/Unitor8 Interfaces by Emagic
the stealth port is only for connecting to old Legacy 1991-1998 serial midi devices
there is some dispute over the idea that these older devices have better timing
explore the threads to find out which one is right for you
http://macos9lives.com/smforum/index.php?board=42.0