Your message refers to 'midi 5 pin or usb'!
I suspect your actual query is nothing to do with this, so why the reference to midi 5 pin or usb?
Your actual stated query seems to relate to using a keyboard mechanism (?) with another unit. I would wonder if there is some connection between your problem and the statement that you're trying to connect a 76 key mechanism to a system designed for 49 keys, and only 49 work? I would rather suspect that something in the inner working of the MX49 is capable of handling 49 keys ONLY, and you would need to change the electronics, or the logic/programming, to get it to see all 76 keys?