The biggest thing about the MIDI interfaces to the serial port is the conversion of RS-232C/EIA-232 -12/+12 voltages to 5V TTL and the electrical isolation by the opto-isolators (photo-couplers) to prevent ground loops.
If not concerned about ground loops you could do the voltage conversions with a couple of transistors and resistors (as with modem interfaces for the 64/128's User Port) as RS-232C/EIA-232 sees logic-low around 3V or so and a logic-high at -3V or less (though many modems accept a direct pull to ground as a negative voltage, and a lot of them are TTL-tolerant.) Better is to use a pairing of 1488 and 1489 ICs or these days the MAX232.
