Tony I might have a temporary fix for you, if you are interested.
I have a Letter "C" from a C128D keyboard. Long story short, I think the keyboard was thrown out a long time ago because I still can't find it. I was going to replace the plunger for the C key, and had that stored away. I've been holding on to this because I hate to dispose of any of this older stuff. I don't see me needing it so you welcome to have it, just cover postage. Doubt it would be very much.
BUT this is why I said It may only be a temp fix.
The key fits fine and functions fine. The print is slighly different. C128D has printing on the Top of the key instead of the Front.