Do an "advanced search" for "completed listings" using the search term "Riedel 24V" on ebay and you will see recent sales. I purchased 5 of the 7 offered - mostly the castings. wish I could have scored the crank assembly !
Did you ever get schematics of the Riedel ? Did you try to run it ? I noticed in one of you pics you fitted a proper carb. Looks to me that the original ran at a constant and high speed - no butterfly in the carb. I have one of these also and recently purchased "most' of an engine from ebay.