MIDI input circuit to trigger 'analog synth voices'

Apologies for any ignorance I'm about to display. Not sure if anyone can help but If I am building a really 'simple' synthesizer which has multiple relaxation transistor oscillators all going into one output, running off a 9v (like look mum no computers circuit), how I would I look into combining a MIDI input so you could control them like synth voices with a musical keyboard, preferably with polyphony. I have looked into building the MIID input circuit with an optocoupler and I think I understand that I need some kind of microcontroller (arduino/teensy?) to convert the digital signals into something that triggers the oscillators? Would the MIDI input circuit work off the 9V? Would I have to decrease the voltage going to the circuit? What other tomfoolery am I going to have to do to get something relativley useful.
There's alot of info on doing this for digital synths but can't find something concise for this project.