You might contact Rodgers and ask for a referral or recommendation.
Here is one product line for adding MIDI control
MIDI 9 - Add MIDI to Any Keyboard Instrument
There are some resources at
MITA International | A World Wide Association of Professional Technicians Organized to Improve the Electronic Music Service Industry
Google MIDI COnversion and also Organ MIDI conversion, and a number of pertinent hits come up.
I don;t know who is still in the business, but there used to be a few companies making MIDI add-on kits.