Several days of soaking in different detergents and a lot of elbow-grease. Also high-pressure water pump with turbo nozzle was used last, but still some of the dirt is hard to get off.
Will try 1-2 days of soaking in a mix of detergents that gave reasonable good result. But so far very happy with the result.
It's alive, well sort of the CPU booted up but no axis movment so I have to search for the fault, looking promising.
Problem solved, found a handwritten note among manual pages about a total reset, so here we go
Now there will be a lot of cleaning job to be done before I can start cutting. Now it is really promising ;)