Tell us about yourself!
I hope this is not something I totally overlooked in your post, like maybe the whole point of your post even, I'm not a super techy person. But is there a simple, minimally-invasive retrofit where the only thing you're adding internally is something the keys can press down on and can be wired to a compact external apparatus that does the rest of work needed to interact with a sequencer? Where the added midi feature even has its own power source and does not i teract with the original keyboards' circuitry. Where the original circuitry doesn't even know it has a midi function being added?
I'm talking before it even needs to reach the midi sequencer.