Hi Ronan,
As a fellow 35 Classic owner the chances are that you have the same portholes as me. They are made by Rutgerson. See https://www.rutgerson.se/product-category/portlights/ I believe you can source replacement acrylic glass though them if you can identify the correct portlight.
When you say the glass is cracked, do you mean it is crazed? Since these are made from acrylic they do craze from the effects of UV sunlight over time. This does not necessarily affect the strength unless the crazing is excessive and is unlikley to be the source of water ingress.
I would tend to agree with Jason above, that the source of the water ingress is probably the sealant bedding down the stainless steel outer frame. Over the past five years I have removed all the windows and re-bedded them with new sealant. I found that Dufour did not use much sealant when they put them in whereas when I put them in I used quite a lot to make sure they won’t let water in again soon.
i hope this helps and good luck.