Don't know enough to help but an issue like that does not sound like it is something for the DIY department. Since you got it for free you are ahead of the game and could still afford to put some money into to get it fixed. If you have a repair shop in the area that is not too expensive you could give that a shot. Or since you already have one in working order you can save your money and apply it towards the purchase of C2 to pair with your M1 and then move your existing C1 to pair with the KM-209.
If you end up sending it to the bone yard let me know. My C1 is missing a cover for one of the buttons and I could use one from yours.