Hopefully doing the “surgery” to the RPi wasn’t to difficult. Less work probably than starting with the RPi Compute Module and getting the connections for an audio interface, the MIDI IO and whatever other interface you need. That would probably be a custom interface board – a carrier board.
Obviously a ton of other options on the software synth side of this, virtual analog, etc. I think this could fill a real need for a lot of people for a powerful no-frills quality keyboard minisynth.