I dont know if its possible to make an arduino midi retrofit kit
It probably is possible but it will not be easy.
At first glance it looks like you will need to wire an Arduino output ( or two ) to every key so the code can simulate pushing a key.
As the keys are probably being scanned in a matrix then the Arduino would need to control an analogue switch to short out the matrix scan just like the key does. That is a lot of wires and a lot of analogue switches.
If you can get a schematic or look at the signals with an oscilloscope then you might be able to simplify this method.
Then you need to simulate the velocity sensitivity of the generator. This is normally acheaved by timing the diferance between a keyboard switch being open when it is first pressed to it been closed at the end of the travel. This would involve using two analogue switches per key.
So while I am sure it could be done it is not a beginner’s project.