I am far from the most experienced keyboard harmonica repair technician on this website, but I would gently clean the reeds with alcohol using a 1-inch paint brush and call it a day. I have done this to several used keyboard harmonicas that I purchased. I am not sure it helped a great deal with the tuning, but it did help me ensure there were no particles or contamination that were binding the reeds against the reed plate. And it made me feel better. I have never seen that much patina on reeds and reed plates before. The above advice was translated to me from an experienced accordion technician. Tune if you must, but as noted in my recent post on tuning, you could have a really good “rough” keyboard harmonica there that would be perfect for certain types of music.