As the original rubber keyboard is not an option, I decided to use the standard 6x6 mini tactile switches. Which can be bought for around €5 per 100 in China.
I already tried as a proof of concept to create the key-caps by printing them in white, spray painting black and then scrape off the paint from the cap-text.
Which works out really nice. Next the caps needed to be modelled so he fit exactly on top of the switch. This took several tries as the fitting needs to be tight but not too tight and so it comes down to a tolerance of +/- 0.1 mm. But in the end this works fine. The caps stay in place, the key-press has a nice tactile feedback and it looks a lot like the original.