There are some really basic serial port to MIDI interfaces available for Amiga. I believe there is at least one design on Aminet.
If I remember correctly (it has been a while for me), standard Amiga serial ports could actually run at MIDI Baud (31250 b/s +-1%). Not sure if the MiniMig can do that or not. If it can, that simplifies the design of an RS-232 to MIDI interface.
Alternatively, lots of Roland sound modules come with a 'MIDI to host' port. This is basically a serial port on the sound module, which accepts MIDI data at standard RS-232 Baud. This requires a (fairly simple) driver though.
Whatever you use, it will have to supported by your sequencer. I'm pretty sure most Amiga sequencers would support serial port MIDI interfaces.
