I've had an old Blasterkey MP3 sitting around for years and using this forum started converting it to work as a stand-alone MIDI keyboard. After watching the videos, and reading comments I realized all it needed was power and the MIDI OUT pin connected to the MIDI IN pin on the male game port on the back of the keyboard. I started connecting the three +5 volt pins together, but decided it may not be necessary. (On my keyboard pins 1 and 8 are connected, but I don't think that is required.) Here is the simplest version I think will work:
Pin 5 ground
Pin 8 +5 volts current limited by a 220Ω resistor (possibly also connected to pin 1)
Pin 12 connected to Pin 15
Then just connect a standard MIDI cable to the MIDI OUT on the keyboard.
This works because the MIDI port on the keyboard is a pass-thru. So the output of the keyboard is being routed to the input so it can pass thru to the existing MIDI port.