At this moment I'm working on a Steingraeber °1896 1m96cm (6'6'') grand. Great piano, if it wasn't for the bridge. The treble bridge shows small cracks all over. The bridgepins didn't move place yet but they might in 5 years, so recapping seems needed. This is a massif one piece beech bridge. I thought of routing till the end of the notches. But how do I use the router? Do I take the soundboard as a base? And do I need a piece of quartersawn beech as the new top, or straight sawn, maybe laminated (where to buy) How do I clamp without a solid ceiling.

Use a planer, not a router. No need to use the board as a guide. Make your new cap thick enough. Just plane evenly and take off a bit more than to the end of the notches.You will trim it to fit later when setting bearing. Do not use beech. Rock maple quarter sawn or a laminate is better. To clamp you can just drill some holes through the bridge and then attach small bolts and fender washers. You can cover up the holes with epoxy later. Just be careful where you drill. Avoid ribs and give yourself access.
There is a series of PTG Journal articles written a few years ago that you should search for and read on this topic.

Go to http://overspianos.com.au/frameset.html
Click on "Technical stuff" along the left-hand side of the page. Then click on "Bridge cap wood..." Right near the bottom of the page that comes up, you'll see a link to "Photogallery of six images..." You'll see a complete guide to how one very knowledgeable rebuilder replaces bridges.

I suggest that you practice on a junk piano.

Also, you should remember that the cracks might have been in this bridge for over a century. They may not be going anywhere in the next five years.

Thanks for the info, link was great.
I decided to fill the cracks with epoxy as they were not that big yet.