As far as I know, Karsten “retired” last year (maybe longer ago now) from supplying big customers like Wilesco/Jensen, and was only doing some private building. I have always had good luck with him, but have heard from others that he wasn’t always reliable.
You can try emailing him direct at gintschel-modellbau@mail.de You will probably have better luck doing that than you would asking Wilesco to build anything custom for you…