thanks to Goemon who sent me a pre-replica-conversion-kit for the VB controller I now have a protoype controller rebuilt (except for the writing “Nintendo Virtual Boy” of course).
He 3d printed the buttons based on scans of real VB buttons (thanks to Retroonyx for the .stl-files).
The grey d-pads are Gameboy pads which I slightly adapted to get the console recognising the inputs (i.e. grinding of the ball on the backside). The feeling is a bit different to the original ones, but I can live with that.
The conversion itself is quite easy since the controller has standard philips screws.
One need to be a bit careful when reassamblying it (e.g. the on/off switch should be placed in the button correctly 😉 ), but besides that one can do this without youtube tutorial videos.
I am very happy with the result 🙂