It looks like I might have to resort to the custom grate option. The standard 19-1/2" grate doesn't have any wiggle room at all for a larger grate, and Killa Grilla doesn't list a replacement grate on the website. I even tried an 18-1/2" kettle cook grate to see if it would fit, but it's too small.
However, I've also overlooked the obvious: ask Weber. I'm not confident that they'll be able to help or if they'll even give me a response, but it's worth a shot.
If I do wind up contacting KG I'm also going to ask about the possibility of a modern 26" grate (25-1/2" diameter) being fitted with extensions so that it can be used in a classic 26" kettle. I've always felt that the distance between the factory grates is a little too small.
EDIT: Emails sent to Weber and Killa Grilla. And so we wait...