http://www.daytonakartbahn.de/Home.aspx were super nice and helped me out with their equipment and knowledge- Thanks for that!
... this is how the rims looked before:
For my new motor I bought another set of cylinder heads - 044 series, already machined for 92/94mm pistons:
The Volksback itself is now equipped with the classic style Radio - and some good working speakers:
I used some MDF board and mounted 2 speakers in the original speaker position: